Of the republic of kazakhstan state university

Contractile vacuoles are found in both marine and freshwater species, but especially in the latter, in which they may discharge as rapidly as every few

seconds. In some species a single vacuole is located near the posterior, but many species possess more than one vacuole (Fig. 2-31С). In Paramecium one vacuole is located at both the posterior and the an­terior of the body (Fig. 2-34F). The vacuoles are al­ways associated with the innermost region of the ectoplasm and empty through one or two perma­nent pores that penetrate the pellicle. The spon­giome contains a network of irregular tubules, which may empty into the vacuole directly or by way of collecting tubules .

When there is more than one vacuole present, they pulsate at different rates depending on their positions. For example, in Paramecium the poste­rior vacuole pulsates faster than the anterior vac­uole because of the large amount of water being delivered into the posterior region by the cytophar­ynx. Although contractile vacuoles may be present in marine species, the rate of pulsation is consid­erably slower than that in freshwater species; they arc probably removing ingested water.

Ciliates differ from almost all other organisms in possessing two distinct types of nuclei—a usually large macronucleus and one or more small micro-nuclei. The micronuclei arc small, rounded bodies and vary in number from l to as many as 20, de­pending on the species. They are diploid, with little RNA. The micronucleus is a store of genetic ma­terial, is responsible for genetic exchange and nu­clear reorganization, and also gives rise to the ma-cronuclei. The macronucleus is sometimes called the vegetative nucleus, since it is not critical in sex­ual reproduction. However, the macronucleus is essential for normal metabolism, for mitotic divi­sion, and for the control of cellular differentiation, and it is responsible for the genie control of the phenotype through protein synthesis.

Figure 2-40 Macronuclei (in gray) of various ciliates (micronuclei, in black). A, Euplotes. B, Vorticella. C, Par­amecium. D, Stentor. (After Corliss, J. О., 1961: The Cil­iated Protozoa. Pergamon Press, N.Y.)

One or more macronuclei are present, and they may assume a variety of shapes (Fig. 2-40). The large macronucleus of Paramecium is somewhat oval or bean shaped and is located just anterior to the middle of the body. In Stentor and Spirostomum the macronuclei are long and arranged like a string of beads. Not infrequently the macronucleus is in the form of a long rod bent in different config­urations, such as а С in Euplotes or a horseshoe in Vorticella. The macronucleus is highly polyploid, the chromosomes having undergone repeated du­plication following the micronuclear origin of the macronucleus. The macronuclei include numerous nucleoli with RNA.

asexual reproduction

Asexual reproduction is always by means of binary fission, which is typically transverse. More accu­rately, fission is described as being homothcto-genic, with the division plane cutting across the ki­netics—the longitudinal rows of cilia or basal bodies (Fig. 2-41Л). This is in contrast to the sym­metrogenic fission of flagellates, in which the plane of division (longitudinal) cuts between the rows of basal granules. Mitotic spindles arc formed only in the division of the micronuclei. Division of the macronuclei is usually accomplished by constric­tion. When a number of macronuclei arc present, they may first combine as a single body before di­viding. The same is true of some forms with beaded or elongated macronuclei.

Modified fission in the form of budding occurs in some ciliate groups, notably the Suctoria. In most members of this subclass the parent body buds off a varying number of daughter cells from the outer surface (Fig. 2-41B); or there is an inter­nal cavity or brood chamber, and the buds form in­ternally from the chamber wall. In contrast to the sessile adults, which lack cilia, the daughter cells, or buds, are provided with several circlets of cilia and are free swimming (Fig. 2-41C). Following a few hours of free existence, the "larva" attaches and assumes the characteristics of the sessile adults.

Although there are no centrioles, the kincto-somes of many ciliates, like the basal granules of flagellates, divide at the time of fission. Further­more, the kinetosomes play a primary role in the re-formation of organelles. It has been found that all of the organelles can be re-formed providing the cell contains a piece of macronucleus and some ki­netosomes. In the more primitive ciliates, in which the cilia have a general distribution over the body surface, the kinetosomes have equal potentials in the re-formation of organelles.

Figure 2-41 A, Homothctogcnic type of fission, in which the plane of division cuts across the kinetics. [After Corliss.) B, Suctorian Ephelota with external buds. (After Noble from Hyman.) C, Detached bud of Dendrocometes. (After Pestel from Hyman.) D, Conjugation in Vorticella. Note the small nonsessilc microconjugant. (After Kent from Hyman.)

An exchange of nuclear material by conjugation is involved in sexual reproduction. By apparently random contact in the course of swimming, two sexually compatible members of a particular spe­cies adhere in the oral or buccal region of the body. Following the initial attachment, there is degener­ation of trichocysts and cilia (but not kinetosomes) and a fusion of protoplasm in the region of contact. Two such fused ciliates are called conjugants; at­tachment lasts for several hours. During this pe­riod a reorganization and exchange of nuclear material occurs (Fig. 2-42Л to F). Only the micron-uclci are involved in conjugation; the macronu­cleus breaks up and disappears either in the course of or following micronuclear exchange.

The steps leading to the exchange of micronu­clear material between the two conjugants are fairly constant in all species. After two meiotic di­visions of the micronuclei, all but one of them de­generate. This one then divides, producing two ga­metic micronuclei that are genetically identical. One is stationary; the other will migrate into the opposite conjugant. The migrating, or "male," nu­cleus in each conjugant moves through the region of fused protoplasm into the opposite member of the conjugating pair. There the "male" and "fe­male" nuclei fuse with one another to form a "zy­gote" nucleus, or synkaryon.

Shortly after nuclear fusion the two animals separate; each is now called an exconjugant. After separation, there follow in each exconjugant a varying number of nuclear divisions, leading to the rcconstitution of the normal nuclear condition characteristic of the species. This reconstitution usually, but not always, involves a certain number of cytosomal divisions. For example, in some forms where there is but a single macronucleus and a sin­gle micronucleus in the adult, the synkaryon di­vides once. One of the daughter nuclei forms a mi­cronucleus; the other forms the macronucleus Thus, the normal nuclear condition is restored without any cytosomal divisions.

A b c d e f g

Figure 2-42 Sexual reproduction in Paramecium caudatum. A to F, Conjugation. В to D, Micronuclei undergo three divisions, the first two of which are meiotic. E, "Male" micronuclei are exchanged. F, They fuse with the stationary micronucleus of the opposite conjugant. G, Exconjugant with macronucleus and synkaryon micronucleus; other mi­cronuclei have been resorbed. (Modified after Calkins from Wichterman.)
However, in Paramecium caudatum, which also possesses a single nucleus of each type, the synkaryon divides three times, producing eight nu­clei. Four become micronuclei and four become macronuclei. The animal now undergoes two cy­tosomal divisions, during the course of which each of the four resulting daughter cells receives one macronucleus and one micronucleus. In those spe­cies that have numerous nuclei of both types, there is no cytosomal division; the synkaryon merely di­vides a sufficient number of times to produce the requisite number of macronuclei and micronuclei.

In some of the more specialized ciliates, the conjugants are a little smaller than nonconjugating individuals, or the two members of a conjugating pair are of strikingly different sizes. Such dioecious macro- and microconjugants occur in Vorticella (Fig. 2-41D) and represent an adaptation for con­jugation in sessile species. The macroconjugant, or "female," remains attached, while the small bell of the microconjugant, or "male," breaks free from its stalk and swims about. On contact with an at­tached macroconjugant the two bells adhere. A synkaryon forms only in the macroconjugant from one gametic nucleus contributed by each conju­gant. However, there is no separation after conju­gation, and the little "male" conjugant degener­ates. In the Suctoria conjugation takes place between two attached individuals that happen to be located side by side.

1. 
   Theory of the origin of multicellular organisms.
   
2. 
   Class calcareous sponge-Calcarea.
   
3. 
   Class Glass sponges - Hyalospongia.
   
4. 
   Class Ordinary sponge - Demospongia. The main representatives of classes of type sponges.
   

Most zoologists agree that metazoans have a com­mon ancestry from some unicellular organism, but there have been differing views as to the particular group of unicellular forms involved and the mode of origin.

Hadzi (1953) and Hanson (1977) have been the chief proponents of a ciliate origin for metazoans. Their theory, which may be called the Syncytial Theory,* holds that multicellular animals arose from a primitive group of multinucleate ciliates. The ancestral metazoan was at first syncytial in structure but later became compartmented or cel-lularized when it acquired cell membranes, which produced a typical multicellular condition. Since many ciliates tend toward bilateral symmetry, pro­ponents of the Syncytial Theory maintain that the ancestral metazoan was bilaterally symmetrical and gave rise to the acoel flatworms, which are therefore held to be the most primitive living meta­zoans. That the acoels are in the same size range as the ciliates, are bilaterally symmetrical, are cil­iated, and tend toward a syncytial condition is con­sidered evidence supporting the primitive position of this group. The ciliate macronucleus, which is absent in acoels, is assumed to have been absent in the multinucleate protociliate stock from which the metazoans arose; according to this theory, it de­veloped later in the evolutionary line leading to the higher ciliates.

There are a number of objections to the Syncy­tial Theory. Nothing comparable to cellularization occurs in the ontogeny of any of these groups, t Furthermore, a ciliate ancestry does not explain the general occurrence of flagellated sperm in metazo­ans. No comparable cells are produced in ciliates, and it is necessary to assume a de novo origin of motile sperm in the metazoan ancestor. The most serious objection to the Syncytial Theory is the ne­cessity for making the acoels the most primitive living metazoans. Bilateral symmetry then be­comes the primitive symmetry for metazoans, and the radially symmetrical cnidarians must be de­rived secondarily from the flatworms. Many spe­cialists now doubt that acoels are even the most primitive flatworms (p. 185).

The Colonial Theory, in which the metazoans are derived by way of a colonial flagellate, is the classic and most frequently encountered theory of the origin of multicellular animals. There is in­creasing evidence in its support, and it is the most widely held view among contemporary zoologists. This idea was first conceived by Haeckel (1874), later modified by Metschnikoff (1887), and revived by Hyman (1940). The Colonial Theory maintains that the flagellates are the ancestors of the meta­zoans, and in support of such an ancestry the fol­lowing facts are cited as evidence. Flagellated sperm cells occur throughout the Metazoa. Flagel­lated body cells commonly occur among lower metazoans, particularly among sponges and cnidar­ians. True sperm and eggs have evolved in the phy-toflagellates. The phytoflagellates display a ten­dency toward a type of colonial organization that could have led to a multicellular construction; in fact, a differentiation between somatic and repro­ductive cells has been attained in Volvox.

Although VoiVox is frequently used as a model for the design of the flagellate colonial ancestor, these autotrophic organisms with plantlike cells are not likely ancestors of metazoans. Ultrastruc-tural evidence points to the choanoflagellates, a small group of animal-like, monoflagellated pro-toza, as the best candidates. Some are solitary and some are colonial.

The Colonial Theory holds that the ancestral metazoan probably arose from a spherical, hollow, colonial flagellate. Like Volvox, the cells were flag­ellated on the outer surface; the colony possessed a distinct anterior-posterior axis and swam with the anterior pole forward; and there was a differentia­tion of somatic and reproductive cells. This stage was called the blastaea in Haeckel's original theory, and the hollow blastula, or coeloblastula, was con­sidered a recapitulation of this stage in the embry-ogeny of living metazoans (Fig. 3-9). According to Haeckel, the blastaea invaginated to produce, a double-walled, saclike organism, the gastraea (Fig. 3-9). This gastraea was the hypothetical metazoan ancestor, equivalent to the gastrula stage in the em­bryonic development of living metazoans.

Blastaea Blastaea

Figure 3-9 Hypothetical stages in the evolution of early metazoans according to Haeckel (left), Metschni­koff (middle), and Grell-Butschli (right). (Greatly modi­fied fromGrell, 1981.)
In addi­tion to embryological evidence, Haeckel noted the close structural similarity between the gastraea and some lower metazoans, such as the hydrozoan cni­darians and certain sponges. Both of these latter or­ganisms are double walled, with a single opening into a saclike cavity.

Haeckel's blastaea and gastraea stages are still widely held as starting points in metazoan evolu­tion and have been recently elaborated in the phylogenetic scheme of Nielsen (1985).

A popular revision of Haeckel's theory still en­countered today was initiated by Metschnikoff (1887), who noted that the primitive mode of gas-trulation in cnidarians is by ingression, in which cells are proliferated from the blastula wall into the interior blastocoel. This produces a solid gastrula. Invagination may have been a secondary embry­onic shortcut. Metschnikoff therefore argued that through the migration of cells into the interior, the originally hollow sphere (blastaea) became trans­formed into an organism having a solid structure (gastraea) (Fig. 3-9). The body of this hypothetical ancestral metazoan is believed to have been ovoid and radially symmetrical. The exterior cells were flagellated and, as such, assumed a locomotor sen­sory function. The solid mass of interior cells func­tioned in nutrition and reproduction. There was no mouth, and food could be engulfed anywhere on the exterior surface and passed to the interior. Since this hypothetical organism is very similar to the planula larva of cnidarians, it has been called the planuloid ancestor.

From such a free-swimming, radially symmet­rical, planuloid ancestor the lower metazoans are believed to have arisen. On the basis of this theory, the primary radial symmetry of the cnidarians can thus be accounted for as being derived directly from the planuloid ancestor. The bilateral symme­try of the flatworms would then represent a later modification in symmetry.

Members of this class, known as calcareous sponges, are distinctive in having spicules con posed of calcium carbonate. All the spicules are of the same general size and are monaxons or three or four-pronged types; they are usually separate.| Spongin fibers are absent. All three grades of structure—asconoid, syconoid, and leuconoid are encountered. Many Calcarea are drab, although briliant yellow, red, and lavender species are known] They are not as large as species of other class most are less than 10 cm in height. Species of сalcareous sponges exist throughout the oceans of the world, but most are restricted to relatively shallow | coastal waters. Genera such as Leucosolenia and Sycon are commonly studied examples of asconoidi and syconoid sponges.

The subclass Sphinctozoa contains a single recently discovered representative (Neocoelia) from | shaded recesses on Indo-Pacific reefs. The Sphinetozoa were abundant from the late Paleozi through the Mesozoic. There are no spicules, but a calcareous skeleton forms an outer perforated wall and also the walls of interior chambers.
2. Class Glass sponges - Hyalospongia

Class Hexactinellida, or Hyalospongiae

Representatives of this class are commonly knows as glass sponges. The name Hexactinellida is rived from the fact that the spicules include a hexaxon, or six-pointed type (Fig. 4-5G). Furthermore, some of the spicules often are fused to form a skeleton that may be lattice-like and built of long, siliceous fibers. Thus, they are called glass spong The glass sponges, as a whole, are the most syi metrical and the most individualized of sponges—that is, they show less tendency to ft interconnecting clusters or large masses with oscula. The shape is usually cup-, vase-, orun and they average 10 to 30 cm in height. The oring in most of these sponges is pale. There is well-developed atrium, and the single osculum is sometimes covered by a sieve plate—a gratelikej covering formed from fused spicules. Lattice-like skeletons composed of fused spicules in sperieJ such as Venus's-flower-basket (Euplectella) n the general body structure and symmetry of thehvl ing sponge and are very beautiful; the white, filmy] skeleton looks as if it were fashioned from wool . Basal tufts of spicule fibers implanted in sand or sediments adapt many species for living on soft bottoms.

The histology of hexactinellids is very different from that of other sponges. All surfaces exposed to water are covered not by pinacoderm but by a syn­cytial layer (trabecular syncytium), through which long spicules may project. Another syncytium, containing flagella with collars, lines the flagellated chambers. Archeocytes are one of the few discrete cell types. The flagellated chambers are commonly thimble shaped and oriented at right angles in par­allel planes to the body wall and central antrium. Hexactinellids are thus somewhat sy­conoid in structure.

In contrast to the Calcarea, the Hexactinellida are chiefly deepwater sponges. Most live between depths of 200 and 1000 meters, but some have been dredged from the abyssal zone. Although found throughout the world, hexactinellids are the dominant sponges in the Antarctic.

Species of Euplectella, Venus's-flower-basket, display an interesting commensal relation with cer­tain species of shrimp (Spongicola). A young male and a young female shrimp enter the atrium and, after growth, are unable to escape through the sieve plate covering the osculum. Their entire life is spent in the sponge prison, where they feed on plankton brought in by the sponge's water cur­rents. A spider crab (Chorilla) and an isopod (Aega) are also found as commensals with some species of Euplectella.
3. Class Demospongiae

This large class contains 90 per cent of sponge spe­cies and includes most of the common and familiar forms. These sponges range in distribution from shallow water to great depths.

Coloration is frequently brilliant because of pigment granules located in the amebocytes. Dif­ferent species are characterized by different colors, and a complete array of hues is encountered.

The skeleton of this class is variable. It may consist of siliceous spicules or spongin fibers or a combination of both. The genus Oscarella is unique in lacking both a spongin and a spicule skel­eton. These Demospongiae with siliceous skele­ton differ from the Hexactinellida in that their larger spicules are monoaxons or tetraxons, never hexaxons. When both spongin fibers and spicules are present, the spicules are usually connected to, or completely embedded in, the spongin fibers.

All Demospongiae are leuconoid, and the ma­jority are irregular, but all types of growth patterns are displayed. Some are encrusting (Fig. 4-8E);

some have an upright branching habit or form ir­regular mounds; others are stringlike or foliaceous (Fig. 4-8C). There are also species, such as Poteiion (Fig. 4-8D), that are goblet or urn shaped, and oth­ers, such as Callyspongia (Fig. 4-8B), that are tu­bular. The great variation in the shapes of the De­mospongiae reflects, in part, adaptations to limitations of space, inclination of substrate, and current velocity. Large upright forms can exploit vertical space and use only a small part of their sur­face area for attachment. Encrusting forms, al­though they require more surface area for attach­ment, can utilize vertical surfaces and very confined habitats, such as crevices and spaces be­neath stones (Fig. 4-2). The largest sponges are members of the Demospongiae; some of the tropi­cal loggerhead sponges (Spheciospongia) form masses over a meter in height and diameter.

Several families of Demospongiae deserve men­tion. The boring sponges, composing the family Clionidae, are able to bore into calcareous struc­tures, such as coral and mollusk shells (Fig. 4-2), forming channels that the body of the sponge then fills. At the surface the sponge body projects from the channel opening as small papillae. These papil­lae represent either clusters of ostia opening into an incurrent canal or an osculum. Excavation, which is begun by the larva, occurs when special amebocytes remove chips of calcium carbonate. The amebocyte begins the process, etching the margins of the chip by digesting the organic frame­work material and dissolving the calcium carbon­ate (Pomponi, 1979) (Fig. 4-14B). The chip is then undercut in the same manner, the amebocyte en­veloping the chip in the process. Eventually, the chip is freed and is eliminated through the excur­rent water canals. Cliona celata, a common boring sponge that lives in shallow water along the Atlan­tic coast, inhabits old mollusk shells. The bright sulfur yellow of the sponge is visible where the bored channels reach the surface of the shell. Cliona lampa of the Caribbean is red, and it com­monly overgrows the surface of the coral or coral­line rock that it has penetrated as a thin encrusting sheet. Boring sponges are important agents in the decomposition of shell and coral (Fig. 4-14).

Members of two families of sponges occur in fresh water, but the family Spongillidae contains the majority of freshwater species. The Spongilli­dae are worldwide in distribution and live in lakes, streams, and ponds where the water is not turbid. They have an encrusting growth pattern, and some are green because of the presence of symbiotic zoochlorellae in the amebocytes. The algae are brought in by water currents and are transferred from the choanocytes to the amebocytes. The growth rate of sponges deprived of zoochlorellae is less than half the normal rate (Frost and William­son, 1980).

Many marine sponges, both Demospongiae and Calcarea, are now known to harbor symbiotic or­ganisms. A few species contain nonmotile dino­flagellates (zooxanthellae), but the most common symbionts are cyanobacteria (blue-green algae), which live within the mesohyl or within special­ized amebocytes. The cyanobacterial symbionts of some keratose sponges, including Verongia, may make up more than 33 per cent of the sponge. Such sponges live in shallow, well-lighted habitats. Ex­cess photosynthate in the form of glycerol and a phosphorylated compound are utilized by the sponge host. Although bacteria filtered from the water currents are an important part of the sponge diet, there is no evidence that the symbiotic bacte­ria are digested (Vacelet, 1979; Wilkinson, 1978, 1979, and 1983).

The family Spongiidae contains the common bath sponges. The skeleton is composed only of spongin fibers. Spongia and Hippospongia, the two genera of commercial value, are gathered from im­portant sponge-fishing grounds in the Gulf of Mex­ico, the Caribbean, and the Mediterranean. (There is no longer any large, commercial sponge fishing in the United States.) The sponges are gathered by divers, and the living tissue is allowed to decom­pose in water. The remaining undecomposed sket-j eton of anastomosing spongin fibers is then washed I (Fig. 4 4K). The colored block "sponges" seen of store counters are a synthetic product.

Class Sclerospongiae

A fourth class of sponges, the Sclerospongiae, con-l tains a small number of species found in grottoj and tunnels associated with coral reefs in various I parts of the world (Jackson et al, 1971). These leu- Г conoid sponges differ from other sponges in having I an internal skeleton of siliceous spicules andspoM gin fibers and an outer encasement of calcium caiT bonate. Гһе numerous oscula are raised on Л calcareous skeletal mass and have a starlike config­uration from the converging excurrent canals | (Hartman and Goreau, 1970).