Of the republic of kazakhstan state university

Nephridia of Annelida

1. What are the three major processes that occur in the digestive system?

2. How does an earthworm process its food? What structure manually breaks down food particles?

3. What is the function of the intestine in all animals? What are the implications of increasing the length (and/or surface area) of the small intestine? How is the surface area of the earthworm intestine increased?

4. What structural feature(s) does the gastrovascular cavity of Dugesia share with the intestine of the earthworm?

5. Why is the digestive tract of Ascaris so unspecialized and why don't tapeworms have one at all?

6. The mouth of the leech has toothed jaws, which it uses to make an incision in its host to feed on its blood. An anticoagulant, hirudin, secreted into the wound keeps the host's blood flowing. What arthropods might benefit from having such an anticoagulant?

Laboratory #14 Origin, structure and evolution of the nervous system of flat, round and annelids.

1. 
   Features of the structure of the nervous system of worms.
   
2. 
   Types of nervous systems of worms.
   

3. 
   Features of the structure of the nervous system of worms.
   
4. 
   Types of nervous systems of worms.
   

Compound microscopes

Dissecting microscopes

Materials

Prepared slides

preserved specimen
Student Prelab Preparation

Before doing this lab, you should read the introduction and sections of the lab topic.

You should use your textbook to review the definitions of the following terms:

clitellum

septum

ventral nerve cord

hearts

nephridia

dorsal blood vessel
You should be able to describe in your own words the following concepts:

How a nerve cell functions

Types of nervous systems found in animals

Major regions of the brain

1. To observe the microscopic structures of nervous system

2. To learn the anatomy of the worms

3. To determine experimentally some of the properties nervous system

During this week of our animal diversity survey, we will study three worm phyla. All of the phyla of worms that we will examine - the annelids, the nematodes, and the platyhelminthes - contain species that are parasites of humans (not to mention other animals and plants). You may already be familiar with some of these creatures: you are likely to encounter leeches (an annelid) simply from wading in a steam or pond, and if you ever had a dog or cat, you probably took it to the vet at least once to be treated for worms (such as roundworms and whipworms, both nematodes, and tapeworms, a platyhelminth).

The parasitic worms that you will examine are for the most part eating and reproducing machines. Consequently, when studying the parasitic worms, take a good look at their digestive and reproductive systems, and then compare them to the digestive and reproductive systems of free-living worms (e.g., earthworms).

1) Phylum Platyhelminthes

The Platyhelminthes include free-living flatworms, like the planarians, and the parasitic tapeworms and flukes. The term flatworm refers to the fact that the body is dorsoventrally flattened. Flatworms are the first organisms to have tissues organized into organs and the first to demonstrate bilateral symmetry. Bilateral symmetry means that one plane passing through the longitudinal axis of an organism divides it into right and left halves that are mirror images. It is characteristic of active, crawling, or swimming organisms and usually results in the formation of a distinct head (cephalization) where accumulation of nervous tissue and sensory structures occurs. This reflects the importance to the organism of monitoring the environment it is meeting
- rather than that through which it has just passed - and results in the presence of definite anterior and posterior ends. The Platyhelminthes and all phyla above them on the evolutionary tree are bilaterally symmetrical or have evolved from bilaterally symmetrical ancestors.

In the Platyhelminthes, different tissues cooperate in any given function. This results in the organ level of organization. Three major sets of organs characterize the phylum. The excretory system consists of flame cells and their associated ducts. The nervous system consists of a pair of anterior ganglia, usually with two nerve cords winning the length of the organism. Nerve cords are interconnected by transverse nerves to form a ladder-like structure. The digestive tract is incomplete (a single opening serves for ingestion of food and elimination of wastes).

The Platyhelminthes are triloblastic and acoelomate. There are three primary germ layers: ectoderm,endoderm, and mesoderm. As with the Cnidaria, the ectoderm gives rise to the outer epithelium, and the endoderm gives rise to the lining of the gut tract The third germ layer, the mesoderm, gives rise to the tissue between the ectoderm and the endoderm, including muscle, excretory structures, and undifferentiated cells referred to as parenchyma. The term acoelomate refers to the fact that them is no body cavity (fluid-filled space) between any of the primary germ layers.

The phylum is divided into four classes:

Class Turbellaria, free-living marine, freshwater, and terrestrial flatworms. Class Trematoda, parasitic internal flukes

Class Cestoda, parasitic tapeworms

Class Monogenea, parasitic external flukes

1. 
   Specimens of Platyhelminthes
   

Obtain a live Dugesia flatworm by sucking it up from the side or bottom of a glass jar using a medicine dropper. Place the specimen in a small Petri dish, making sure that it is completely covered by pond water, and examine it under your dissecting scope.

Dugesia is a common turbellarian (= planarian) that resides in freshwater steams and ponds. Note your animal's shape, pigmentation, and mode of locomotion. Dugesia, as well as most free-living flatworms, move over surfaces by means of cilia on their ventral surface. Note the pigmented eye spots, or ocelli, located on the triangular "head" of the animal. These eye spots are sensitive only to light and dark, and are unable to resolve images. On either side of the eye spots are lateral lobes which serve as chemosensory organs. Cover your culture dish (top and sides) with a piece of aluminum foil, and place the dish on a dark background with a microscope light shining on it. After 5 to 10 minutes, remove the foil and observe where your animal is relative to the light. Is your animal positively or negatively attracted to light (= phototactic)? How might this behavior be adaptive for the animal in its natural environment?
B) Dugesia, microscope slide (Figure 2).

1. Observe a prepared whole mount of Dugesia under low power of your compound microscope. You should see the eye spots and the "brain", nerve cord.

2. The cerebral ganglia, a bilobed structure beneath the eye-spots, that appears as a slightly lighter area.

3. From the cerebral ganglia two longitudinal nerve cords pass backward, and several smaller nerves pass off in front. Ex¬amine the specimen by reflected light, looking particularly at the nervous system and pharynx. What relation have the nerve cords behind?

4. With the high power and good light, look for the water-vascular tubules. The region anterior to the cerebral ganglia is a favorable place. They form a clear, branching tracery, a little lighter than the surrounding tissue. The flicker of the flame cells can usually be seen, but they may be more easily seen in Crossobolhrium Examine chart and text-book figures of the water-vascular system.

Make a good-sized drawing of a worm, showing the above points.

Figure 2: Dugesia, whole mount (from Stamps, Phillips & Crowe. The laboratory: a place to do science, 3rd ed.)



C) Clonorchis, Class Trematoda, preserved specimen and microscope slide (Figure 3).

Clonorchis sinensis, the human liver fluke, is a parasitic trematode found in the bile ducts of humans. Like most parasitic worms, the life cycle of C. sinensis is extremely complex and involves several hosts. The adult worm sheds eggs into the bile ducts of its human host, which eventually reach the small intestine and are passed with feces. If the eggs are ingested by the proper species of aquatic snail, they hatch into larvae that then progress through a series of asexual stages, culminating in an infective larval stage known as cercariae. The cercariae are ciliated, and have a tail for swimming. They pass out of the snail, and then briefly swim about in the water until they encounter a fish. Then the cercariae penetrate the muscles of the fish, lose their tails, and remain encysted until the fish is eaten by the definitive (= final host). These encysted larvae are freed in the human small intestine after consumption of improperly prepared fish. The immature flukes migrate through the bile duct and its tributaries throughout the liver, where they develop into adult worms. If untreated, an infection by Clonorchis can lead to enlargement and cirrhosis of the liver.

Observe a prepared slide of Taenia under low power of your compound microscope. Your specimen, Taenia pisiformis, is a tapeworm of carnivores (notably, dogs), and closely resembles T. solium and T. saginata, common parasites of humans contracted by eating poorly prepared beef or pork, respectively. Tapeworms share many features with flukes, including an outer cuticle, attachment structures, expansive reproductive organs, and complex life cycles involving intermediate hosts. Unlike flukes, however, tapeworms lack a mouth and gastrovascular cavity, a consequence of their life in vertebrate organs of high nutritional activity (i.e., the small intestine). Bathed by food in their host's intestine, they absorb predigested nutrients across their body surface via diffusion and possibly, active transport.

The body of a tapeworm is divided into four main regions. A small scolex ("head") bears suckers and an elevated rostellum with curved hooks; the suckers are used for attachment to the host's organs. Immediately posterior to the scolex is a "neck" that produces many proglottids ("segments") by asexual budding. Each proglottid is potentially a complete reproductive unit containing by male and female reproductive organs (i.e., each is hermaphroditic). Why might hermaphroditism be especially advantageous for an internal parasite?

The second region consists of small, immature proglottids nearest to the neck and scolex.

The third region, or mid-section, consists of mature proglottids, each with well-developed male and female reproductive organs. These proglottids engage in internal, cross fertilization. In a mature proglottid, locate the lateral genital pore that contains both a thin, tubular vagina and a stouter vas deferens. Trace the vagina posteriorly and note that it passes between two ovaries and terminates at a shell gland anterior to a yolk gland. Eggs are fertilized and "yolked" before passing anteriorly into a sac-like uterus. The male reproductive system consists of numerous small, round testes, each with a tiny tubule that connects to a single vas deferens, which transports sperm to the genital pore.

The fourth and posterior region of the tapeworm consists of gravid (= "pregnant") proglottids. In gravid proglottids, most of the gonads are atrophied, leaving only an enlarged uterus packed with eggs. These gravid proglottids eventually break off from the body of the adult worm, and pass out of the digestive tract in the host's feces. When a small mammal, such as a rabbit, ingests a proglottid or eggs, the eggs hatch into larvae that then bore through the intestinal wall and then move through the circulatory system where they eventually become encysted in muscle tissue. When the rabbit is eaten by a dog, the encysted larvae are released, and develop into adult worms. As can be seen from the specimen on display, tapeworms can be quite large: T. solium, a parasite of the human intestine, can reach a length of 10 feet!

2) Phylum Nematoda

Nematodes are probably the most abundant and ubiquitous animals on earth, having invaded virtually every habitat. Most of the approximately 10,000 species of nematodes are free-living, but many are parasites of animals, including humans. Trichinella spiralis, for example, is contracted by eating insufficiently cooked pork. The adult worms develop in the human intestine, releasing larvae which move through the lymphatic system, eventually ending up in muscle tissues where they encyst. Other nasty nematode parasites of humans include Necator americanus (hookworms) and Wuchereria, which results in elephantiasis. Nematodes also are parasites of plants and can cause enormous crop damage; as a result, some large universities have departments of plant pathology devoted to the study of plant pathogenic nematodes.

1) they are triploblastic.

2) they have a pseudocoelom, a cavity incompletely lined by mesodermally-derived tissue.

3) the fluid-filled pseudocoelom functions as a hydrostatic skeleton.

4) they have a complete, one-way digestive tract, having both a mouth and an anus.

5) they have a non-living, protective cuticle covering their bodies.

We will examine preserved specimens of Ascaris lumbricoides, commonly known as the roundworm, an intestinal parasite of humans. Humans contract Ascaris by ingesting eggs from the soil. Once ingested, the eggs hatch, releasing larvae. The larvae bore through the small intestine and migrate via the venous and lymphatic systems to the lungs. There the larvae continue to grow, and pass through several larval stages. After a few weeks, the larvae are coughed-up, literally, and then swallowed, where they develop into mature adults in the small intestine.

Examine preserved specimens of male and female ascarids. The male is smaller, and has a curved, posterior end for grasping the female during copulation. These differences in size and morphology are examples of sexual dimorphisms. Why do you think sexes of Ascaris differ in size?

Obtain an Ascaris worm from your laboratory TA. Female Ascaris are somewhat easier to dissect, because their larger size makes it easier to find and identify various organs. However, you should examine both a dissected male and female worm, so ask around in lab to find a dissected worm of the opposite sex. Determine the dorsal surface by locating the anus, which is on the ventral side. Then, place the animal in a dissecting pan, pinning it at both the head and tail ends, dorsal side up. Using fine scissors or a scalpel, carefully cut along the midline of the dorsal surface to expose the internal organs. Pin the body wall back so that organs are exposed, and submerge your animal in water so that its internal organs float freely.

The straight, tubular digestive system for the most part is undifferentiated (why?) and consists of a mouth, pharynx, intestine, and anus.

The most conspicuous organs in the pseudocoel are the tubular reproductive organs. Nematodes are very prolific, and females of some species may shed thousands of eggs daily. Carefully uncoil the reproductive organs, which are Y-shaped. The vagina is located at the base of the Y, and the two arms are the uteri. Each uterus connects to an oviduct, which in turn connects to an ovary. The uterus, oviduct, and ovary are continuous and have no obvious demarcations between them, although the uterus tends to be slightly larger in diameter.

3) Phylum Annelida

The phylum Annelida includes approximately 15,000 marine, freshwater, terrestrial, and parasitic species. It is the archetypal 'wormy' phyla, with the majority of forms possessing a long, thin shape. The long shape is attained in annelids by metameric segmentation, a linear repetition of body parts and organs. Segmentation has enabled annelids to become particularly adept at a particular type of locomotion, burrowing. In addition to segments, other annelid features include:

1) A triploblastic, bilaterally-symmetric body plan with a true coelom; that is, their body cavity is completely lined by mesodermally-derived tissue (the peritoneum).

2) The fluid-filled coelom functions as a hydrostatic skeleton.

3) A closed circulatory system with dorsal and ventral blood vessels, with one to many "hearts"; often with hemoglobin as a respiratory pigment.

4) A nervous system including a cerebral ganglion (= brain).

5) An excretory system consisting of nephridia.

The phylum is divided into three classes, two of which are characterized by tiny bristles (setae) in their body walls:

Class Polychaeta (= many setae), marine species such as sandworms that usually possess fleshy, lateral extensions (parapodia) from their body wall.

Class Oligochaeta (= few setae), freshwater and terrestrial species (e.g., earthworms).

Class Hirudinea, leeches, which lack setae and move in an inch-worm fashion using anterior and posterior suckers, or swim via undulations.

Earthworm dissection: Obtain a preserved specimen of the earthworm (Lumbricus) for dissection. Identify the dorsal and ventral surfaces. Make an incision on the dorsal surface from the prostomium (mouth) to the middle of the body. Carefully cut and pin back the skin to expose the internal anatomy. Use Figure 8 to identify the structures listed below, and consider the basic function of each structure as you examine it.

You should be able to identify the following structures on a dissected earthworm:

clitellum

septum

seminal vesicle

hearts

cerebral ganglia (“brain”)

dorsal blood vessel

Observe the specimens of Hirudo, a leech representative of the class Hirudinea. Leeches probably evolved from oligochaetes, and are the most specialized of annelids. Some leeches are predaceous, but most are external parasites of other animals, and have several adaptations for a parasitic lifestyle. Their body is dorso-ventrally flattened, and the first and last segments are modified to form suckers. Why is a flat shape useful for ectoparasites? Can you think of any arthropod parasites that have flat shapes? Except in primitive species, the internal segmentation has been lost. Consequently, the movement of leeches differs somewhat from other annelids, and depends on the use of suckers for attachment to the substrate. Given what you know about locomotion in earthworms, how do you think a leech uses its suckers to move about? The mouth of the leech has toothed jaws, which it uses to make an incision in its host to feed on its blood. An anticoagulant, hirudin, secreted into the wound keeps the host's blood flowing. What arthropods might benefit from having such an anticoagulant? Like oligochaetes, leeches are hermaphroditic, bearing both male and female reproductive organs.

Anterior sucker/mouth Intestine



INFORMATION PROCESSING AND SENSORY INPUT

The bodily processes of multicellular organisms need to be coordinated, and to do this, most animals have a nervous system, which enable them to coordinate and regulate the activities of different body parts via rapid electrical signaling. The simplest type of nervous system consists of simple sensors, which receive information, effectors such as muscles and glands, which carry out responses to stimuli, and a system of nerves that run between them. Some organisms, like cnidarians (e.g., Hydra), can exist with a simple network of neurons since their

lives are spent attached to the substratum, and responses to their environment do not need to be well coordinated.

For animals that move about to locate food or mates, more-sophisticated sensory detection and coordination of responses is necessary. In these animals, we see the development of simple to complex brains (or ganglia, which are accumulations of nerve cell bodies, in the case of invertebrates), well-defined sensory systems, and developed effectors (like muscles). In addition, the nerves that run between these components of the nervous system become increasingly more developed from simple to complex organisms. When you look at the brains (ganglia) in the specimens over the next several weeks, you should notice where they are located and how complex they are. You should also look at some of the sensors (such as eyes), the effectors, and the nerves that run between them and the ganglia in these specimens. In particular, pay attention to where the major nerves are located (are they on the dorsal side or the ventral side?).