Of the republic of kazakhstan state university

The body of flukes is oval to elongate, usually not more than a few cm long, and the mouth is typi­cally at the anterior end. Adhesive suckers are usu­ally present around the mouth and may also be present midventrally. The monogenetic flukes pos­sess large posterior attachment organs, called opisthaptors, provided with various structures, such as suckers and hooks (Fig. 7-25).

In contrast to the epidermis of the turbellarian, the body of a trematode is covered a nonciliated cytoplasmic syncytium, the tegument, overlying consecutive layers of circular, longitudi­nal, and diagonal muscle. The syncytium repre­sents extensions of cells that are located in the pa­renchyma (Fig. 7-26).

The mouth leads into a muscular pharynx that pumps into the digestive tract the cells and cell fragments, mucus, tissue fluids, or blood of the host on which the parasite feeds. The pharynx passes into a short esophagus and one or, more commonly, two blind intestinal ceca that extend posteriorly along the length of the body (Fig. 7-27A). The physiology of nutrition is still incom­pletely understood, but secretive and absorptive cells have been reported, so digestion is apparently extracellular in part.

The tegument plays a vital role in the physiol­ogy of flukes. It provides protection, especially against the host's enzymes in gut-inhabiting spe­cies. Nitrogenous wastes are passed to the exterior through the tegument, and it is the site of gas ex­change. In endoparasites the tegument absorbs some amino acids. The protein synthesis involved in fluke egg production and in larval reproduction places especially heavy demands on the amino acid supply.
Nematoda.

The nematode cuticle is considerably more com­plex than that of other aschelminths. It contains collagen, as well as other compounds, and it is or­ganized within three main layers (Fig. 9-14). The outer cortical layer is bounded externally by a thin epicuticle, which may exhibit quinone tanning. It is typically annulated (ringed). The median layer varies from a uniform granular structure in some species to the occurrence of struts, skeletal rods, fi­brils, or canals in others. The basal layer may be striated or laminated or contain spiral fibers.

Growth in nematodes is accompanied by four molts of the cuticle. Beginning at the anterior end, the old cuticle separates from the underlying epi­dermis and a new cuticle is secreted, at least in part. The old cuticle is shed in fragments or intact. Molting does not occur after the worm becomes adult, but the cuticle continues to grow.

The epidermis, also called hypodermis, is usu­ally cellular but may be syncytial in some species. A striking feature of the nematode epidermis is the expansion of the cytoplasm into the pseudocoel along the middorsal, midventral, and midlateral lines of the body (Fig. 9-15). The bulging epider­mis thus forms longitudinal cords that extend the length of the body. The epidermal nuclei are com­monly restricted to these cords and are typically ar­ranged in rows.

The muscle layer of the body wall is composed entirely of longitudinal, obliquely striated fibers arranged in bands, each strip occupying the space between two longitudinal cords. The fibers may be relatively broad and flat, with the contractile fila­ments limited to the base of the fiber, or they may be relatively tall and narrow, with filaments at the base and sides. In both types the base of the cell containing the contractile fiber is located against the hypodermis, and the side of the cell with the nucleus is directed toward the pseudocoel (Fig. 9-16). Each nematode muscle fiber possesses a slender arm that extends from the fiber to either the dorsal or the ventral longitudinal nerve cord, where innervation occurs (Fig. 9-15). In most ani­mals a nerve process extends from the nerve cord to the main body of the muscle.

The nematode pseudocoel is spacious and filled with fluid. No free cells are present, but fixed cells, located either against the inner side of the mm layers or against the wall of the gut and the inte organs, are characteristic of many nematodes.

The polychaete epidermis, or integument, is composed of a single layer of cuboidal or columnar epithelium, which is covered by a thin collagen cu­ticle (see Fig. 10-9 for evolution of cuticle). Mucus-secreting gland cells are a common compo­nent of the epithelium.

Beneath the epithelium lie, in order, a layer of circular muscle fibers, a much thicker layer of lon­gitudinal muscle fibers, and a thin layer of perito­neum. Although the muscles of the body wall essentially comprise two sheaths, the longitudinal fibers typically are broken up into four bundles—two dorsolateral and two ventro­lateral.

Within the spacious coelom, the gut is sus­pended by septa and mesenteries. Thus, each coelomic compartment is divided into right and left halves, at least primitively. However, the septa have partially or completely disappeared in many polychaetes.

The structure and histology of the oligochaete body wall, especially in terrestrial species, is essen­tially like that of burrowing polychaetes. A thin cu­ticle overlies an epidermal layer, which contains mucus-secreting gland cells. Circular muscles are well developed, and the septa partitioning the coe­lom are relatively complete. Earthworms, which have the best developed septa, may possess sphinc­ters around septal perforations to control the flow of coelomic fluid from one segment to another.

In most earthworms each coelomic compart­ment, except at the extremities, is connected to the outside by a middorsal pore located in the interseg­mental furrows and provided with a sphincter. These pores exude coelomic fluid, which aids in keeping the integument moist. When disturbed, some giant earthworms squirt fluid several centimeters.

Body Wall and Coelom of Hirudinea.

The body wall contains a more distinct connective tissue dermis than is present in other annelids, and some of the unicellular gland cells of the integu­ment are very large and sunken into the connective tissue layer (Fig. 10-63). The longitudinal muscle layer of the body wall is powerfully developed, but there are also circular, oblique, and dorsoventral muscle fibers.

1. 
   System of metabolism in flat, round and annelids.
   
2. 
   The structure of the digestive system of flat, round and annelids.
   

1. 
   System of metabolism in flat, round and annelids.
   

Both free-living and parasitic platyhelminths utilize oxygen when it is available. Most of the parasitic platyhelminths studied have a predominantly anaerobic metabolism (i.e., not dependent upon oxygen). This is true even in species found in habitats—such as the bloodstream—where oxygen is normally available.

Parasitic platyhelminths are made up of the usual tissue constituents—protein, carbohydrates, and lipids—but, compared to other invertebrates, the proportions differ somewhat; i.e., the carbohydrate content tends to be relatively high and the protein content relatively low. In larval and adult cestodes, carbohydrate occurs chiefly as animal starch, or glycogen, which acts as the main source of energy for species in low oxygen habitats. The level of glycogen, like other chemical constituents, can fluctuate considerably, depending on the diet or feeding habits of the host. In some species, more than 40 percent of the worm’s dried weight is glycogen.

Because carbohydrate metabolism is important in parasitic flatworms, a substantial amount of carbohydrate must be present in the host diet to assure normal growth of the parasite. Hence the growth rate of the rat tapeworm (Hymenolepis diminuta) is a good indicator of the quantity of carbohydrate ingested by the rat. Experiments have shown that most parasitic worms have the capability of utilizing only certain types of carbohydrate. All tapeworms that have been studied thus far utilize the sugar glucose. Many tapeworms can also utilize galactose, but only a few can utilize maltose or sucrose.

An unusual constituent of both trematodes and cestodes is a round or oval structure called a calcareous corpuscle; large numbers of them occur in the tissues of both adults and larvae. Their function has not yet been established, but it is believed that they may act as reserves for such substances as calcium, magnesium, and phosphorus.

The chief proteins in cestodes and trematodes are keratin and sclerotin. Keratin forms the hooks and part of the protective layers of the cestode egg and the cyst wall of certain immature stages of trematodes. Sclerotin occurs in both cestode and trematode eggshells, especially in those that have larval stages associated with aquatic environments.

Platyhelminth eggs hatch in response to a variety of different stimuli in different hosts. Most trematode eggs require oxygen in order to form the first larval stages and light in order to hatch. Light is thought to stimulate the release of an enzyme that attacks a cement holding the lid (operculum) of the egg in place. A similar mechanism probably operates in cestodes (largely of the order Pseudophyllidea) whose life cycles involve aquatic intermediate hosts or definitive hosts, such as birds or fish.

In many cestodes, especially those belonging to the order Cyclophyllidea, the eggs hatch only when they are ingested by the host. When the host is an insect, hatching sometimes is apparently purely a mechanical process, the shell being broken by the insect’s mouthparts. In vertebrate intermediate hosts, destruction of the shell depends largely on the action of the host’s enzymes. Activation of the embryo within the shell and its subsequent release depend on other factors, including the amount ofcarbon dioxide present, in addition to the host’s enzymes. Factors involving a vertebrate host are also important in establishing trematode or cestode infections after encysted or encapsulated larval stages have been ingested. Under the influence of the same factors, tapeworm larvae are stimulated to evaginate their heads (i.e., turn them inside out, so to speak), a process that makes possible their attachment to the gut lining.

True gills occur in only a few oligochaetes>| cies of the aquatic genera Dero and Aulophorus have a circle of finger-like gills at the posterior of the body. A tubificid, Branchiura, has filamentous gills located dorsally and ventrally in the posterior quarter of the body.

The larger oligochaetes usually have hemoglobin dissolved in the plasma. The hemoglobin in Lumbricus transports 15 to 20 percent of oxygen utilized under ordinary burrow condition| where the partial pressure of oxygen is ah same as that in the atmosphere above ( When the partial pressure drops, the hemoglobin compensates by increasing its carrying capari| (Weber, 1978).

Many aquatic oligochaetes tolerate relative low oxygen levels and, for a short period, eve complete lack of oxygen. Members of the far Tubificidae, which live in stagnant mud and 1 bottoms, are notable examples. There are members of this family, such as Tubifex tubiex, from long exposure to ordinary oxygen tens Tubiex ventilates in stagnant water by щ its posterior end out of the mud and waving about.

The glossiphoniids and piscicolids (rhynchobdellids) have retained the blood-vascular system of oli­gochaetes, but the coelomic sinuses act as a supplemental circulatory system. In the other leech orders the ancestral circulatory system has disappeared, and the coelomic sinuses and fluid have been converted to a blood-vascular system. The hemocoelomic fluid is propelled by the con­tractions of the lateral longitudinal channels.

Gills are found only in the Piscicolidae, the gen­eral body surface providing for gas exchange in other leeches. The piscicolid gills are lateral leaflike or branching outgrowths of the body wall.

Respiratory pigment (extracellular hemoglobin) is found only in the gnathobdellid and pharyngo-bdellid leeches and is responsible for about half of the oxygen transport.

The mouth leads into a muscular pharynx that pumps into the digestive tract the cells and cell fragments, mucus, tissue fluids, or blood of the host on which the parasite feeds. The pharynx passes into a short esophagus and one or, more commonly, two blind intestinal ceca that extend posteriorly along the length of the body. The physiology of nutrition is still incom­pletely understood, but secretive and absorptive cells have been reported, so digestion is apparently extracellular in part.

The blind-ending intestine of trematodes consists of a simple sac with an anterior or midventral mouth or a two-branched gut with an anterior mouth; an anus is usually lacking, but a few species have one or two anal pores. Between the mouth and the intestine are often a pharynx and an esophagus receiving secretions from glands therein. The intestine proper, lined with digestive and absorptive cells, is surrounded by a thin layer of muscles that effect peristalsis; i.e., they contract in a wavelike fashion, forcing material down the length of the intestine. In many larger flukes lateral intestinal branches, or diverticula, bring food close to all internal tissues. Undigested residue passes back out of the mouth.

Cestodes have no digestive tract; they absorb nutrients from the host across the body wall. Most other flatworms, however, have conspicuous digestive systems.The digestive system of turbellarians typically consists of mouth, pharynx, and intestine. In the order Acoela, however, only a mouth is present; food passes directly from the mouth into the parenchyma, to be absorbed by the mesenchymal cells.

Free-living platyhelminths (class Turbellaria), mostly carnivorous, are particularly adapted for the capture of prey. Their encounters with prey appear to be largely fortuitous, except in some species that release ensnaring mucus threads. Because they have developed various complex feeding mechanisms, most turbellarians are able to feed on organisms much larger than themselves, such as annelids, arthropods, mollusks, and tunicates (e.g., sea squirts). In general, the feeding mechanism involves thepharynx which, in the most highly developed forms, is a powerful muscular organ that can be protruded through the mouth. Flatworms with a simple ciliated pharynx are restricted to feeding on small organisms such as protozoans and rotifers, but those with a muscular pharynx can turn it outward, thrust it through the tegument of annelids and crustaceans, and draw out their internal body organs and fluids. Turbellarians with a more advanced type of pharynx can extend it over the captured prey until the animal is completely enveloped.

Digestion is both extracellular and intracellular. Digestive enzymes (biological catalysts), which mix with the food in the gut, reduce the size of the food particles. This partly digested material is then engulfed (phagocytized) by cells or absorbed; digestion is then completed within the gut cells.

PARASITIC FORMS

In the parasitic groups with a gut (Trematoda and Monogenea), both extracellular and intracellular digestion occur. The extent to which these processes take place depends on the nature of the food. When fragments of the host’s food or tissues other than fluids or semifluids (e.g., blood and mucus) are taken as nutrients by the parasite, digestion appears to be largely extracellular. In those that feed on blood, digestion is largely intracellular, often resulting in the deposition of hematin, an insoluble pigment formed from the breakdown of hemoglobin. This pigment is eventually extruded by disintegrating gut cells.

Despite the presence of a gut, trematodes seem able to absorb glucose and certain other materials through the metabolically active tegument covering the body surface. Tapeworms, which have no gut, absorb all nutrients through the tegument. Amino acids (the structural units of proteins) and small molecules of carbohydrate (e.g., sugars) cross the tegument by a mechanism called active transport, in which molecules are taken up against a concentration gradient. This process, similar to that in the vertebrate gut, requires the expenditure of energy. Cestodes may also be able to digest materials in contact with the tegument by means of so-called membrane digestion, a little-understood process.

Type Nemathelminthes

Class Nematoda

Many free-living nematodes are carnivorous and feed on small metazoan animals, including nematodes. Other species are phytophagous, marine and freshwater species feed on algae, and fungi. Algae and fungi is also important food sources for many terrestrials species, but there are fungi that trap nematodes. The worms are caught when they pass through cial hyphal (threadlike) loops, which close on luxation. A large number of terrestrial nematode pierce the cells of plant roots and suck out tents. Such nematodes can be responsible for ous damage to commercial plants. There many deposit-feeding marine, freshwater, and restrial species, which ingest substratum particles. Deposit feeders and the many nematodes that live on dead organic matter, such as dung, or on the de­composing bodies of plants and animals, feed only on associated bacteria and fungi, however. This is true of the common vinegar eel, Turbatrix aceti, which lives in the sediment of nonpasteurized vin­egar, Nematodes are the largest and most ubiqui­tous group of organisms feeding on fungi and bac­teria and are of great importance in the food chains leading from decomposers.

The mouth of the nematode opens into a buccal cavity, or stoma, which is somewhat tubular and lined with cuticle. The cuticular surface is often strengthened with ridges, rods, or plates, or it may bear a large num­ber of teeth. The structural details of the buccal cavity are correlated with feeding habits and are of primary importance in the identification of nematodes. Teeth are especially typical of car­nivorous nematodes; they may be small and nu­merous or limited to a few, large, jawlike processes

The feeding habits of Mononchus papillatus, which is a toothed nematode, have been described by Steiner and Heinly (1922). This ter­restrial carnivore, which has a large dorsal tooth opposed by a buccal ridge, consumes as many as 1000 other nematodes during its life-span of ap­proximately 18 weeks. In feeding, this nematode attaches its lips to the prey and makes an incision in it with the large tooth. The contents of the prey are then pumped out by the pharynx.

In some carnivores, as well as in many species that feed on the contents of plant cells, the buccal capsule carries a long hollow or solid spear (stylet), which can protrude from the mouth.

Both kinds of stylet are used to puncture prey,« the hollow stylet may act as a tube through v the contents of the victim are pumped out. In; let-bearing herbivore, it is used to penetn root cell walls, being thrust rapidly forward i. backward. Both groups secrete рЛ ryngeal enzymes that initiate digestion of the p or the plant cell contents and may even aidi penetration of the plant cell wall.

The buccal cavity leads into a tubular pharymJ referred to as the esophagus by nematologists. The pharyngeal lumen is tnradiateinn section and lined with cuticle. The* is composed of myoepithelium (as in gastroti and gland cells (Ruppert, 1982). Frequently,! pharynx contains more than one muscular sweM or bulb. The pharynx or pharyngeal bulbs act] pumps and bring food from the mouth intotht| testine. Valves are frequently present.

From the pharynx a long tubular intestine соя posed of a single layer of epithelial cells cxtcndii| length of the body. A valve located at eache the intestine prevents food from being forced, of the intestine by the fluid pressure of the f docoel. A short, cuticle-lined rectum (cloaca in tl male) connects the intestine with the anus, i is on the midventral line just in front of the j rior tip of the body.

Digestive enzymes are produced by the ph geal glands and the intestinal epithelium, 1 tion begins extracellularly within the intt lumen but is completed intracellular (Deua 1978).

Type Annelida

Class Polychaeta.

Nutrition

The feeding methods of polychaetes are closely correlated with the various life-styles of the class.

Raptorial FEEDERS

Raptorial feeders include members of many fami­ly ot surface-dwelling species, many pelagic groups, tubicolous eunicids and onuphids, and gal­ley dwellers like the glvcerids and nephtyids. The prey consists of various small invertebrates, including other polychaetes, which are usually cap­tured by means of an eversible pharynx (proboscis). The pharynx commonly bears two or more horny jaws composed of tanned protein. The pharynx is rapidly everted; this places the jaws at the anterior of the body and causes them to open. The food is seized by the jaws and the pharynx is retracted. Although protractor muscles may be present, an increase in coelomic pressure resulting from the contraction of body wall muscles is an im­portant factor in the eversion of the pharynx. When pressure on the coelomic fluid is reduced, the pharynxHs withdrawn by the retractor muscles, which extend from the body wall to the pharynx .

Raptorial tube dwellers may leave the tube par­tially or completely when feeding, depending on the species. Diopatra uses its hood-shaped tube as a lair. Chemoreceptors monitor the ventilating current of water passing into the tube, and when approaching prey is detected, the worm partially emerges from the tube opening and seizes the victim with a complex pharyngeal armature of teeth. During feeding the prey may be clasped with the enlarged anterior parapodia. Species of Diopa­tra may also feed on dead animals, algae, organic debris, and small organisms, such as forams, that are in the vicinity of the tube or become attached to it.

Some raptorial feeders, such as syllids and glycerids, have long, tubular proboscises. Species of Glycera live within a gallery system constructed in muddy bottoms. The system contains numerous loops that open to the surface. Lying in wait at the bottom of a loop, the worm can de­tect the surface movements of prey such as small crustaceans and other invertebrates, by changes in water pressure. It slowly moves to the burrow opening and then seizes the prey with the proboscis.

When the proboscis is retracted, it occupies ap­proximately the first 20 body segments. At the back of the proboscis are four jaws arranged equi-distantly around the wall. The proboscis is at­tached to an S-shaped esophagus. No septa are present in these anterior segments, and the probos­cis apparatus lies free in the coelom. Just prior to eversion of the proboscis the longitudinal muscles contract violently, sliding the proboscis forward and straightening out the esophagus. The proboscis is then everted with explosive force, and the four jaws emerge open at the tip. Each jaw contains a canal that delivers poison from a gland at the jaw base.

Nutrition

The majority of oligochaete species, both aquatic and terrestrial, are scavengers and feed on dead or­ganic matter, particularly vegetation. Earthworms feed on decomposing matter at the surface and may pull leaves into the burrow. They also utilize or­ganic material obtained from mud or soil that is in­gested in the course of burrowing. The food source and feeding habits of earthworms are related to the species zonation described in the previous section.

Fine detritus, algae, and other microorganisms are important food sources for many tiny, fresh­water species. The common, minute Aeolosoma collects detritus with its prostomium. The ciliated ventral surface of the prostomium is placed against the substratum, and the center is el­evated by muscular contraction. The partial vac­uum dislodges particles, which are then swept into the mouth by cilia. Members of the genus Chaeto-gaster, little oligochaetes that are commensals on freshwater snails, are raptorial and catch amebas, ciliates, rotifers, and trematode larvae by a sucking action of the pharynx.

The digestive tract is straight and relatively simple (Fig. 10-52). The mouth, located beneath the prostomium, opens into a small buccal cavity, which in turn opens to a more spacious pharynx. The dorsal wall of the pharyngeal chamber is mus­cular and glandular and forms a bulb or pad, which is the principal ingestive organ. In aquatic forms the pharynx is everted and the mucus-covered mus­cular disc collects particles on an adhesive pad (Fig. 10-51). In earthworms the pharynx acts as a pump. Pharyngeal glands produce a salivary secretion con­taining mucus and enzymes.

The pharynx opens into a narrow, tubular esophagus, which may be modified at different lev­els to form a gizzard or, in lumbricid earthworms,

a crop. In some forms there are two to ten gizzards, each occupying a separate segment. The gizzard, which is used for grinding food particles, is lined with cuticle and is very muscular. The crop is thin walled and acts as a storage chamber.

A characteristic feature of the oligochaete gut is the presence of calciferous glands in certain parts of the esophageal wall. When highly developed, the glandular region becomes completely separated from the esophageal lumen and may appear exter­nally as lateral or dorsal swellings (Fig. 10-52). The calciferous glands are involved in ionic regulation rather than digestion. They function in ridding the body of excess calcium taken up from food. The calcium is excreted into the esophagus as calcite, which is not absorbed in transit through the intestine.

The intestine forms the remainder of the diges­tive tract and extends as a straight tube through all but the anterior quarter of the body. The anterior half of the intestine is the principal site of secretion and digestion, and the posterior half is primarily absorptive. In addition to the usual classes of diges­tive enzymes, the intestinal epithelium of earth­worms, at least, secretes cellulase and chitinase. The absorbed food materials are passed to blood si­nuses that lie between the mucosal epithelium and the intestinal muscles. The surface area of the in­testine is increased in many earthworms by a ridge or fold, called a typhlosole, which projects inter­nally from the middorsal walls.

Surrounding the intestine and investing the dorsal vessel of oligochaetes is a layer of yellowish cells, called chloragogen cells, which play a vital role in intermediary metabolism, similar to the role of the liver in vertebrates. Chloragogen tissue is the chief center of glycogen and fat synthesis and stor­age. Deamination of proteins, the formation, monia, and the synthesis of urea also take pk these cells. In terrestrial species silicates obi from food material and the soil are removed fi the body and deposited in the chloragogen cells waste concretions.

Nutrition

Leeches possess either a proboscis or a sucking pharynx and jaws. The proboscis (order Rhynchob­dellida) is an unattached tube lying within) boscis cavity, which is connected to the vol mouth by a short, narrow canal proboscis is highly muscular, has a m lumen, and is lined internally and exten cuticle. Ducts from large, unicellular glands open into the proboscis. When feet animal extends the proboscis out of the i forcing it into the tissue of the host.

In jawed leeches (order Gnathobdcllida), lack a proboscis, the mouth is located in prior sucker. Just with mouth cavity are three large, oval, bladlike each bearing a large number of small teeth the edge. The three jaws are arranged in a ma4 one dorsally and two laterally. When the Г feeds, the anterior sucker is attached to the surf of the prey or host, and the edges of the jaw through the integument. The Wswing toward and away from each other, ac-mated by muscles attached to their bases. Salivary gbdssecrete an anticoagulant called hirudin.

Immediately behind the teeth, the buccal cavity opens into a muscular, pumping pharynx. The er-pobdellids also have a pumping pharynx, but the ware replaced by muscular folds.

The remainder of the digestive tract is relatively uniform throughout the class. A short esophagus opens into a relatively long stomach, or crop. The stomach may be a straight tube, as in the crpobdcl-to,but more commonly it is provided with 1 to 11 pairs of lateral ceca (Fig. 10-61). Following the stomach is an intestine, which may be a simple mbeor, as in the rhynchobdellids, may have tour pairs of slender lateral ceca. The intestine opens into a short rectum, which empties to the outside though the dorsal anus, located in front of the pos­terior sucker.

Many leeches are predacious, but about three fourths of the known species are bloodsucking ec­toparasites. However, in many cases the difference lies only in the size of the host. The Hirudinidae especially demonstrate a gradation from predation to parasitism. The Erpobdellidae contain the great­est number of predacious leeches, but this type of feeding habit is found in other families as well. Predatory leeches always feed on invertebrates. Prey includes worms, snails, and insect larvae. Feeding is relatively frequent, and the prey is usu­ally swallowed whole. Many glossiphoniids suck all the soft parts from their hosts and are best re­garded as specialized predators. In laboratory stud­ies Erpobdella punctata consumed 1.78 tubificids (oligochaete worms) per day and Helobdella stag-nalis 0.57 per day (Cross, 1976).

The bloodsucking leeches attack a variety of hosts. Some, primarily species of Glossiphonia and Helobdella, feed only on invertebrates, such as snails, oligochaetes, crustaceans, and insects, but vertebrates are hosts for most species. Piscicolidae are parasites of both freshwater and marine fish, sharks, and rays. The glossiphoniids feed on amphibians, turtles, snakes, alli­gators, and crocodiles. Species of the cosmopolitan glossiphoniid genus Theromyzon attach to the nasal membranes of shore and water birds. The aquatic Hirudinidae and the terrestrial Haemadip-sidae feed primarily on mammals, including hu­mans (Fig. 10-59G).

Parasitic leeches are rarely restricted to one host, but they are usually confined to one class of vertebrates. For example, Placobdella will feed on almost any species of turtles and even alligators, but they rarely attack amphibians or mammals. On the other hand, mammals are the preferred hosts of Hirudo. Furthermore, some species of leeches that are exclusively bloodsuckers as adults are preda­cious during juvenile stages.

The mammalian bloodsuckers, suchasHfj| on contacting a thin area of the host's intej attach the anterior sucker very tightly to M and then slit the skin. The jaws of HirM about two slices per second. The incisioafl thetized by a substance of unknown origji pharynx provides continual suction, andthcM tion of hirudin prevents coagulation of tbf Penetration of the host's tissues is notwdl stood in the many jawless, proboscis-bearj cies that are bloodsuckers. The proboscis b rigid when extended, and it is possible! tration is aided by enzymatic action.

Leech digestion is peculiar in a numbd spects. The gut secretes no amylases, lipases,! dopeptidases. The presence of only ехорерг* perhaps explains the fact that digestion iJ sucking leeches is so slow. Also character)! the leech gut is a symbiotic bacterial florae important in nutrition. In both the bloodsi medicinal leech Hirudo medicinalis and the|a dacious Erpobdella nctoculata. the gut Meters responsible for a considerable part of digdl they may be significant in the digestion leeches. The bacterium Pseudomonashin of Hirudo medicinalis breaks down I lar-weight proteins, fats, and carbohydrate^ the bacterial population increases signihe. lowing the ingestion of blood by the leech (I 1975). The bacteria may also produce vital other compounds that are used by theleecJ

Bloodsuckers feed infrequently, but** do, they can consume an enormous qua blood. Haemadipsa may mgest ten times nsi weight, and Hirudo two to five times it’s a weight. Following ingestion, water is removed e blood and excreted through the nephridia. (digestion of the remaining blood cells then epbee very slowly. These leeches can then tol-t long periods of fasting. Medicinal leeches :en reported to have gone without food for one years, and since they may require 200 :st a meal, they need not feed more than a year in order to grow.

1. 
   Structure and origin of the excretory system of flat, round and annelids.
   
2. 
   Circulatory and respiratory systems nemertine and annelids.
   

Flukes, like other flatworms, have protonephri­dia, and there is typically a pair of longitudinal col­lecting ducts. There may be two anterior, dorsolat­eral nephndiopores (in Monogenea) or a single posterior bladder and nephridiopore (in Trematoda. In the ectoparasites, the proto­nephridia are probably only osmoregulatory in function. The function of the protonephridia in en-doparasites is still uncertain.

Excretion

Protonephridia are absent in all nematodes and) patently disappeared with the ancestral mere of the class. Some nematodes have no special excretory system, but many do possess a peculiar system of gland cells, with or without tubules, that has some excretory function. In the class Adenophorea, which includes most marine and freshwa­ter nematodes, there is usually one large gland cell, called a renette gland (Fig. 9-19A), located ventrally in the pseudocoel near the pharynx. The gland cell is provided with a necklike duct that opens ventrally on the midline as an excretory pore.

All members of the class Secernentea, which in­cludes many terrestrial species, have a more spe­cialized tubular system, still composed of only a few cells. Three long canals are arranged to form an H (Fig. 9-19B). Two are lateral and extend inside the lateral longitudinal cords. The two lateral ca­uls are connected by a single transverse canal, from which a short, common, excretory canal leads to the excretory pore, located ventrally on the mid­line. In many nematodes, that part of each lateral canal anterior to the transverse canal has disap­peared, so the system is shaped like a horseshoe; in others the tubules on one side have been lost, so the system is asymmetrical.

The excretory gland cell or tubules are known to eliminate foreign substances, but may have other functions as well. Ammonia is the principal nitrogenous waste of nematodes and is removed through the body wall and eliminated from the digestive system along with the indigestible residues.

Type Annelida.

Class Polychaeta.

METANEPHRIDIA

The most common type of excretory organ among coelomate animals is a metanephridium. In contrast to the blind protonephridial tubule, a metanephridial tubule opens internally into the coelom. The opening is often funnel-like and clothed with ciliated perito-num, in which case it is called a nephrostome. In imsegmented coelomates there may be one nephri­te or one to several pairs of metanephridia; in seg­mented groups, such as the annelids, the metane­phridia are serially repeated, one pair per segment.

In general, a metanephridium processes coelomic hid. Blood filtrate passes into the coelom at various sites of filtration, depending on the species. For ex­ample, in a mollusk part of the heart wall is the major ate of filtration and is composed of podocytes, cells with finger-like processes that interdigitate [Fig. 11-90]. The slits between processes are the sites of titration. Podocytes are found at the filtration sites of many animals, e.g., the glomeruli of the ver­tebrate kidney

Coelomic fluid, derived from blood filtrate, passes through the nephrostome into the ciliated nephridial tubule. Here it becomes modified by selective reab-sorption and secretion, and the product is finally ex­pelled through the nephridiopore as urine. The extent of tubular secretion and reabsorption depends in pert on the environment in which the animal lives, i.e., whether it is an osmoconformer or osmoregula-tor. The tubule wall is correspondingly specialized and provided with a vascular backing.

Excretion

Polychaete excretory organs are either protone­phridia or metanephridia (Box 10-2). In primitive polychaetes there is one pair of nephridia per seg­ment, but reduction to few or even one pair for the entire worm has occurred in some families. The an­terior end of the nephridial tubule is located in the coelom of the segment immediately anterior to that from which the nephridiopore opens (Fig. 10-2). The tubule penetrates the posterior septum of the segment, extends into the next segment, where it may be coiled, and then opens to the ex­terior in the region of the neuropodium. Both the preseptal portion of the nephridium and the pos-tseptal tubule are covered by a reflected layer of peritoneum from the septum.

Protonephridia of a type called solenocytes are found in phyllodocids, alciopids, tomopterids, gly-cerids, nephtyids, and a few others. The soleno­cytes are always located at the short preseptal end of the nephridium and are bathed by coelomic fluid. The solenocyte tubules are very slender and delicate and arise from the nephridial wall in bunches (Fig. 10-37) Each tubule contains a single flagellum, and the wall is composed of parallel rods connected by the thin lamellae. The latter represent the fenestrations through which fluid passes; this arrangement is characteristic of other types of protonephridia.

All other polychaetes possess metanephridia, in which the preseptal end of the ne­phridium possesses an open, ciliated funnel, the nephrostome, instead of solenocytes. Typical me­tanephridia are found in the nereids, where the nephrostome possesses an outer invest­ment of peritoneum and the interior is heavily cil­iated. The postseptal canal, which extends laell next successive segment, becomes greatly coded form a mass of tubules, which are enclosed J thin, saclike covering of peritoneal cells. CoiliJ probably an adaptation that increases the игШ area for tubular secretion or rcabsorption. TkJ phridiopore opens at the base of the neuropcdJ on the ventral side. The entire lining of thetutJ is ciliated.

The metanephridia of most other polycbl differ only in minor details (Fig. 10-38) bill display various degrees of regional restncwl the more specialized families. In the fan worn! where only one pair of functional nephndul main, the two nephridia join at the midline to Л a single median canal, which extends forwrJ open through a single nephridiopore on tbet. Excretory waste is deposited directly ouuide, and fouling of the tube is avoided.

In polychaetes the association of the blood vcs-Klswith the nephridia is variable. The fan worms HKt the arenicolids lack a well-developed nephri­dial blood supply, and the coelomic fluid must be the principal route for waste removal. In other po-hxhaetes the nephridia are surrounded by a net­work of vessels. In the nereids the nephridial blood supply is greater in those species that live in brack-lb water.

Many polychaetes, particularly nereids, can tol­erate low salinities and have become adapted to life mbrackishsounds and estuaries. The gill (notopo-dullobe) of Nereis succinea contains cells special­ly fot absorbing ions. A small number of species hve in fresh water. The sabellid Manayunkia spe-:wexample, occurs in enormous numbers in ctrum regions of the Great Lakes, such as around themouth of the Detroit River. There are a few ter-Шіаі polychaetes, all tropical Indo-Pacific ner­eids, which burrow in soil or live in moist litter.

Chloragogen tissue, coelomocytes, and the in­testinal wall may play accessory roles in excretion. Chloragogen tissue is composed of brown or green­ish cells located on the wall of the intestine or on various blood vessels. Chloragogen tissue, which has been studied much more extensively in earth­worms (see p. 316), is an important center of inter­mediary metabolism and hemoglobin synthesis.

The adult oligochaete excretory organs are meta­nephridia, and typically, there is one pair per seg­ment except at the extreme anterior and posterior ends. In the segment following the nephrostome, the tubule is greatly coiled, and in some species, such as Lumbricus, there are several separate groups of loops or coils. Before the ne­phridial tubule opens to the outside, it is some­times dilated to form a bladder. The nephridio­pores are usually located on the ventrolateral surfaces of each segment.

In contrast to the majority of oligochaetes, which possess in each segment a single, typical pair of nephridia called holonephridia, many earth­worms of the families Megascolecidae and Glos-soscolecidae are peculiar in possessing additional nephridia, which are multiple or branched. Either typical or modified nephridia may open to the out­side through nephridiopores, or they may open into various parts of the digestive tract, in which case they are termed enteronephric. A single worm may possess a number of different types of these nephridia, each being restricted to certain parts of the body.

Earthworms excrete urea, but they are less per­fectly ureotelic than are other terrestrial animals. Although urea is present in the urine of Lumbricus and other earthworms and although the level of urea depends on the condition of the worm and the environmental situation, ammonia remains an im­portant excretory product.

Salt and water balance, which is of particular importance in freshwater and terrestrial environ­ments, is regulated in part by the nephridia (Fig. 10-54B). The urine of both terrestrial and fresh­water species is hypoosmotic, and considerable reabsorption of salts must take place as fluid passes through the nephridial tubule. Some salts are also actively picked up by the skin.

In the terrestrial earthworms water absorption and loss occur largely through the skin. Under nor­mal conditions of adequate water supply, the ne­phridia excrete a copious hypoosmotic urine. It is not certain whether reabsorption by the ordinary nephridia is of importance in water conservation, but the enteronephric nephridia do appear to rep­resent an adaptation for the retention of water. It passing the urine into the digestive tract, muchl the remaining water can be reabsorbed as it goes through the intestine. Worms with enteronepmi systems can tolerate much drier soils or do J have to burrow so deeply during dry periods.

A few aquatic oligochaete species are capabled encystment during unfavorable environmenrd conditions. The worm secretes a tough, mucca covering that forms the cyst wall. Some specie form summer cysts for protection against desicr*! tion; others form winter cysts when thewatertet perature becomes low.

During dry seasons or during the winter, eaii worms migrate to deeper levels of the soil, dm 3 meters in the case of certain Indian species.Ц moving to deeper levels, an earthworm often » dergoes a period of quiescence and in drypeJ may lose as much as 70 per cent of its water.№ ance is restored and activity resumed as soon» water is again available.

Excretion

Leech contain 10 to 17 pairs of metanephridia, ill the middle third of the body, one pair segment. As a result of the coelom ton and the loss of septa in the leech body, ihndial tubules arc embedded in connective the nephrostomcs project into the coc-c channels. Each nephrostome opens into a itedcapsule.

In most leeches the cavities of the capsule and idial canal do not connect, and the two of the nephridium may even have lost ictural connection. Many branching, intra-ilar canals drain into the nephridial canal, rpens to the outside through the ventrola-nephndiopore. Secretion into the intracellular iculiisthe initial source of nephridial fluid, Btfleunne is very hypoosmotic to the blood, in-inung reabsorption of salts. The nephridia are important organs of osmoregulation (Haupt,

The function of the nephridial capsules is be-Btdtobe the production of coelomocytes. The coelomocytes are phagocytic and engulf particulate matter, but the eventual fate of the waste-laden cells is not certain. They may migrate to the epi­dermis or to the epithelium of the digestive tract. Particulate waste is also picked up by botryoidal and vasofibrous tissue of the hirudinid leeches and by pigmented and coelomic epithelial cells of glos-siphoniids and piscicolids.
2. Circulatory and respiratory systems nemertine and annelids.

Internal Transport

In most polychaetes there exists a well-developed blood-vascular system, in which the blood is en­closed within vessels. The basic plan of circulation is relatively simple. Blood flows anteriorly in a dor­sal vessel situated over the digestive tract; at the anterior of the body, the dorsal vessel is connected to a ventral vessel by one to several vessels or by a network of vessels passing around the gut. The ven­tral vessel carries blood posteriorly beneath the al­imentary tract.

In each segment the ventral vessel gives rial one pair of ventral, parapodial vessels, *һісЦ ply the parapodia, the body wall, and the ni and to several ventral, intestinal vessels that the gut. The dorsal vessel, intl receives a corresponding pair of dorsal para| vessels and a dorsal intestinal vessel. The and ventral parapodial vessels and the dorsal ventral intestinal vessels are interconnected network of smaller vessels.

There are many variations of this basic tory pattern, and the circulatory mechani polychaetes are not nearly so uniform as tL scription might suggest. All polychaetes relv »| varying degrees on the transporting capacity coelomic fluid, and some have lost the blood-cular system completely.

Although the polychaete system is usually a doled system, in that blood is restricted to vessels, lithe vessels tend to be relatively thin walled and ootalwayslined with endothelium (Nakao, 1974). Moreover, the endothelium, when present, is myoendothelium, and the basal membrane is di­ked toward the lumen instead of away from it, ts in vertebrates (Fig. 10-36Л) [Ruppert and Carle, 1983).

The gills are usually provided with afferent and efferent vascular loops permitting a two-way flow. This is true, for example, of the gills of lugworms mdthe branchial, notopodial lobes of nereids (Fig. 1Ш|. On the other hand, the radioles of fan worms, which function in both food gathering and (asexchange, contain only a single vessel, within whichblood flows tidelike, in and out. In many po-khaetes, such as glycends, the gills are irrigated nth coelomic fluid and not blood.

In general, blood is driven by peristaltic waves of contraction that sweep over the blood vessels, particularly the dorsal vessels. The vessel wall in Уфа. for example, consists of a single layer of myoepithelial cells that contain striated myofibrils ananged in a circular direction or in both circular and longitudinal directions (Boilly and Wissocq, І97Л. Many polychaetes have accessory, heartlike pumps located in various places within the blood-rascular system.

The blood contains few cells compared with coelomic fluid. In small polychaetes it is usually colorless, but in larger species and those that bur­row in soft bottoms, the blood contains respiratory pigments dissolved in the plasma. In fact, within the Polychaeta are found three of the four rcspira-torv pigments of animals. Hemoglobin is the most common of these pigments, but chlorocruorin is characteristic of the blood of the serpulid and sa-belhd fan worms and also of the Flabclligcridac and Ampharetidae. Chlorocruorin is an iron porphyrin like hemoglobin, but the slight difference in side chunsgives it a green rather than a red color. There I little reason for separating chlorocruorin from pksmahemoglobin, which it resembles more than ether resembles intracellular hemoglobin. Mage-he has a blood-vascular system with anuclcatcd corpuscles containing a third iron-bearing protein pgment (not a porphyrin) called hemerythrin. Hemerythrin is a protein similar to hemocyanin rather than a porphyrin, and the molecule of O. is earned between two iron atoms.

Plasma, or extracellular, hemoglobin and chlo­rocruorin molecules are always very large. The piamahemoglobin of Arenicola. for example, con-urns % heme units. The entire molecule attains a molecular weight of 3,000,000. This compares with a molecular weight of 60,000 in mammalian hemoglobin, in which there are four hemes, each attached to a 15,000 molecular weight unit. There are numerous polychaetes, including Glycera, Cap-itella, and some terebellids, in which the blood-vas­cular system is reduced or absent and the coelomic fluid functions in internal transport. The coelom of these worms contains hemoglobin located in coe­lomic corpuscles. Such coelomic hemoglobin, like the corpuscular hemoglobin of vertebrates, is al­ways a small molecule.

Mangum (1985) believes that Hb packed in red blood cells may be the primitive condition in ani­mals and has been retained in various animal groups, including some polychaetes. The more spe­cialized extracellular condition appears to be re­lated to the lack of capillary beds in most poly­chaetes. In the larger vessels through which polychaete blood is pumped, the blood is less vis­cous with its Hb in solution than it would be with red blood cells.

There are a number of interesting exceptions to the usual disposition of respiratory pigments just described. The blood of Serpula contains both he­moglobin and chlorocruorin. Most terebellids and ophelids possess not only coelomic red corpuscles but also a blood-vascular system with a different hemoglobin. The two hemoglobins are not alike. The coelomic hemoglobin (Hb) of Amphitrite has a greater affinity of O, at low oxygen tensions (dis­sociation curve to the left) than does the blood-vas­cular Hb. This difference facilitates the passage of oxygen from the blood-vascular system to the coe­lomic fluid, which is the principal source of oxygen for internal tissues.

In the majority of polychaetes the respiratory pigments function in oxygen transport, although for only a part of the oxygen consumed. When the blood from the gills does not become mixed with unoxygenated blood before delivering its oxygen load to the target tissues, the oxygen affinity of the hemoglobin is relatively low (oxygen dissociation curve to the right). This is the situation for poly­chaetes like Amphitrite and the fan worms (e.g., Sabella), in which the gills are at the anterior end. In worms with segmental gills, in which the blood from the gills is mixed with unoxygenated blood en route to the target tissues, the oxygen affinity of the hemoglobin is high; i.e., the hemoglobin holds on to its oxygen at relatively low oxygen tensions .

1. 
   Life cycles of flat, round and annelids.
   
2. 
   The structure of the larvae of flat, round and annelids
   

The class Trematoda (digenetic flukes) is the largest group of parasitic flatworms. Over 6000 q have been described, and new descriptions are continually being published. There are many species that cause parasitic diseases in man and domesticated animals.

In contrast to the monogenetic trematodes, life cycles of the digenetic trematodes involve two to four hosts. The host for the adult is the definitive host, and the one to three hosts for the numerous developmental stages are termed intermediate hosts. The adhesive organs are typically two suckers. One sucker, called the oral sucker, is located around the mouth. The other sucker, the acetabulum, is located ventrally in middle or posterior end of the body.

Most digenetic trematodes are endoparasirk The definitive hosts include all groups of vertebrates, and virtually any organ system may be infected. The intermediate hosts arc largely invertibrates, commonly snails.

The life cycle is complex and will be introduced by a generalized scheme followed by more specific examples. The egg is enclosed within an oval shell with a lid, deposited in the gut, and passed to outside with the definitive host's feces. A snail may ingest an egg containing a miracidium or the ciliiated, free-swimming miracidium hatched from the egg, or the larval stage may penetrate the snail's epidermis. It thus comes to inhabit| the hemocoel.

Inside the snail the miracidium, which loses its cilia when it enters the host, begins a second developmental stage, called a sporocyst. Inside the hollow sporocyst, germinal cells give rise to a number of embryonic masses. Each mass develops into another developmental stage, called a redia or daughter sporocyst, which is also a chambered form. Germinal cells within redia again develop into a number of larvae called cercariae. The term digenetic refers to this second generation of individuals, pro­duced asexually.

The cercaria, a fourth developmental stage, pos­sesses a digestive tract, suckers, and a tail. The cercaria leaves the host and is free swimming. Its sec­ond intermediate host may be an invertebrate (commonly an arthropod) or a vertebrate, in which it encysts. The encysted stage is called a metacercaria. If the host of the metacercaria is eaten by the final vertebrate host, the metacercaria escapes from its cyst, migrates, and develops into the adult form within a characteristic location in the host.

Excysting metacercaria Encysted mafl| in fish

Consumption of infected fish
Figure 7-32 The Chinese liver fluke, Opisthorchis sinensis: A, Dorsal view of adult worm. B, Life cycle.
Class Cestoda (Cestoidea) Life Cycles

Tapeworms are endoparasites in the guts of verte­brates. Their life cycles require one, two, or some­times more intermediate hosts, which are arthro­pods and vertebrates. The basic developmental stages are an oncosphere larva, which hatches free the egg , and a cysticercus or plerocercoid si which is terminal and develops into an adult though the following few examples illustrate the basic life cycle patterns of tapeworms, varieties exist.

Species of the family Taeniidae are among the best known tapeworms. Taeniarhynchus saginata, the beef tapeworm, is one of the most common species in humans, where it lives in the intestine and frequently reaches a length of over 3 meters. Proglottids containing embryonated eggs are eliminated through the anus, usually with feces. If an infected person defecates in a pasture, the eggs may be eaten by grazing cattle, sheep, or goats. On hatching in the intermediate host, an on­cosphere larva, bearing three pairs of hooks, bores into the intestinal wall, where it is picked up by the circulatory system and transported to striated mus­cle. Here the larva develops into a cysticercus stage. The cysticercus, sometimes called a bladder worm, is an oval worm about 10 mm in length, with the scolex invaginated. If raw or in­sufficiently cooked beef is ingested by humans, the cysticercus is freed, the scolex evaginates, and the larva develops into an adult worm in the gut.

A severe infection of adult tapeworms may cause diarrhea, weight loss, and reactions to the toxic wastes of the worm. The worms may be elim­inated with drugs. Much more serious is cysticer­cus infection. Fortunately, the cysticercus stage of the beef tapeworm will not develop within hu­mans, but this is not the case for the pork tape­worm, Taenia solium, and for the dog tapeworm, Echinococcus granulosus. The adult Echinococcus, which lives in the intestine of a dog, is minute, with only a few proglottids present at any one time. Many different mammals, including humans, can act as intermediate hosts, although herbivores are the most important in completing the life cycle. The cysticerci of the pork tapeworm develop in subcutaneous connective tissue and in the eye, brain, heart, and other organs. The bladder worm, or hydatid, of Echinococcus develops mostly in the lung or liver but can develop in many other sites as well. The bladder worms of both species can be very dangerous when growing in such places as the brain and can do much damage elsewhere. Hydatid cysts can reach a large size and contain a great vol­ume of fluid (up to many liters), which if released into the host can cause severe reactions. Bladder worm cysts can be removed only by surgery.

Figure. Structure and life cycle of the beef tapeworm, Taeniarhynchus saginatus. (Adapted from various sources)

The female nematode lays eggs inside the host, which are then most often passed out in the host's feces. At this point, the eggs hatch in the external environment. Cues such as moisture levels or temperature trigger the larva inside the egg to begin producing enzymes that will dissolve the membrane of the egg. In response to the right conditions, the larva then also pushes on the weakened membrane to break through.

Once hatched, the nematode larvae begin to eat bacteria and grow. Some types of nematode larvae that didn't leave the initial host as an egg, such as viviparous filarial worms, are transferred to an intermediate host at this time, usually through the bite of an fly or other arthropod. Or, as in the case of Strongyloides stercoralis, they might be passed out in the feces as stage 1 larvae. When the larvae cannot grow anymore because of the size of their cuticle (skin), they must molt. To molt, they develop a new cuticle under the old one and then shed the former cuticle. The first molt marks the transition of a larva from stage 1 (L1) to stage 2 (L2).