Zoology Lab exercise No. 5 - Platyhelminthes & Nematoda

Kingdom Animalia

Phylum Platyhelminthes

Class Turbellaria

Class Trematoda

Class Cestoda

Phylum Nematoda

Other pseudocoelomates

Note: - Bring you text book , biology drawing paper and hard lead pencil to lab with you.

A. Phylum Platyhelminthes.

This phylum includes the flatworms which are characterized by having three body layers: ectoderm, mesoderm and endoderm. They are thus triploblastic and have achieved the tissue level of organization. In lab we will be examining members of the classes Turbellaria, Trematoda and Cestoda.

1. Class: Turbellaria

This is a large group organized into as many as nine orders and hundreds of species. Locally we have a few small, difficult to find terrestrial flatworms but aquatic species are common in springs, streams and even caves. They are easily collected from the underside of rocks in the New River. Obtain a slide of "Planaria, digestive system". It will be difficult to observe many internal organs from these slides due to the dark pigmentation of the animal's body wall. The gastrovascular cavity will show up prominently, however, due to the fact that these specimens were fed a dark stained material prior to their preservation. You will be able to see the muscular pharynx which is used both for food gathering and for the egestion of non-absorbed materials. How does the gastrovascular cavity of Planaria differ from that of the Hydra?

What is the advantage of a branched digestive system?

Obtain a slide of a cross-section of a planarian and identify as many structures as you can. Figure 19-4 (p. 419) in your text is a cross-section, but you will not be able to see everything it shows. You will, however, see the dark, thickened epidermis with rhabdites, the diffuse parenchyma making up most of the interior, and the structure of the large pharynx located centrally. The label "Pharynx" on Fig. 19-4 is misleading in that the guide line points to the interior cavity, or lumen; actually the pharynx includes everything shown inside the pharyngeal cavity. In the cross section, the "intestines" may be difficult to locate at first as they will not be exactly as illustrated in the stylized text figure. If you are not certain you have located them, ask your instructor for help. Make a sketch of the whole planarian (low power) showing the digestive system and pharynx and a second sketch (on the same page) of a cross-section indicating where (on the whole specimen) you believe the section was taken. On the cross-section, label endoderm, mesoderm and ectodermal regions and any other structures you can identify.

Obtain a live planarian and place it in a small dish. Cover it with a thin film of water.

Look carefully in the pharyngeal cavity. How many mouths does your specimen have? ________ Observe some basic behavior of your planarian and make notes: How does it move? Does it change shape as it moves? Can it bend its body as it turns? What must be the internal muscular arrangement to allow such bending? Can it back up? How can this be accomplished? With a probe, carefully roll it over. Can it right itself? How is this accomplished? Which end rolls over first - head end or tail end? Shine a bright light directly on the planarian. Does it avoid the light, move toward the light or ignore the light?

2. Class: Trematoda

Owing to their highly specialized lifestyle as internal parasites, adult trematodes (flukes) do not have ciliated bodies as do the turbellarians. Instead, there is a fairly thin cuticle which can be treated to make it transparent. Entire specimens can easily be cleared and stained for making whole mounts.

Obtain a prepared slide of Clonorchis sinensis and examine with low power under a compound microscope or high power under a dissecting microscope to get an idea of general body form. Some of the internal organs will be readily visible. We can get an idea of how much more complex the anatomy of flatworms is when compared with cnidarians. Make an outline sketch of the body about 6 inches long (Get the proportion right. How wide is it relative to its length? Where is it widest? ) centered on the page. This will allow space for providing internal structures later on. It is a good idea to sketch in lightly the position of the larger structures at this point. You can add details as you examine individual structures.

The digestive system is very simple. In contrast to that of turbellarians, the mouth is located at the anterior (smaller) end and leads directly into a thick, circular pharynx (appears to have two lobes). Although the mouth is centered in what is called the oral sucker, it is obvious that the worm cannot attach itself and eat at the same time. Just behind the pharynx is a short central esophagus which divides into an intestine running along each side of the body. Note that each intestinal branch, although a straight, simple tube, ends blindly: no anus. There is less dense parenchyma in flukes, and diffusion is more efficient, so that highly branched intestines are not as necessary as in turbellarians.

The excretory system removes only the molecular-sized dissolved wastes of various metabolic processes, chiefly dissolved gasses, salts, nitrogenous compounds and is thus functionally analogous to the urinary system of higher animals. Starting at the posterior end of the body you can locate the excretory pore, which opens internally into a long, pale central excretory bladder lying between the two intestinal branches. At its anterior end, which seems to be rather blunt, you may see that a branch goes toward each side of the body and may be traced, running parallel and just lateral to the intestine, as far as the ventral sucker. Eventually these two branches break down into much smaller ones, finally branching down to the level of the flame cells which are the actual excretory organs (not visible in a whole mount).

As with most parasites, a very high premium is placed by flukes on egg production. Since completion of the life cycle (Fig. 19-8) is so uncommon, vast numbers of eggs are produced constantly by the adult. If the first larval stage, the miracidium, is able to find and invade a snail, there is another reproductive stage, the sporocyst, which can also increase the numbers of the later redial and cercarial stages. Greater numbers enhance the chance for survival. This, however, requires huge energy expenditures for reproduction. Most flukes are hermaphroditic. What is the advantage of this?

The male reproductive system is simple although much of it can not be found on a whole mount: the obvious parts are the two large, highly branched testes, centrally located in the posterior third of the animal. Each testis is drained by a fine tube, the vas efferens, which you may not be able to see. These tubes join to form the seminal vesicle which runs forward and empties to the outside on the front edge of the ventral sucker.

The female reproductive system is more complicated, and dominated by the highly coiled and looped single uterus which occupies much of the central part of the body. It often looks black because of the dark-shelled eggs it contains. The uterus opens to the outside through the genital pore on the front edge of the ventral sucker, and you should be able to trace its course right down to this pore. Going back the other way, near the middle of the body the uterus suddenly becomes smaller and not so packed with eggs; this region could be identified as the oviduct. Just behind this region you can see two oval objects. One is nearly clear and is the seminal receptacle which stores sperm received during copulation and deals them out very slowly to fertilize the thousands of eggs that will be produced. Just in front of, or sometimes superimposed on, the seminal receptacle, is a darker reddish object, the ovary. Just at the point where the oviduct leaves the ovary, you can see one or more pinkish, finger-like objects which are the shell glands.

Now looking again at the edges of the body just lateral to the intestinal branches, you can locate some very fine, diffuse, pink-colored glands. These are the vitelline glands (= yolk glands). Each is connected to the oviduct by a very fine yolk duct which may or may not be visible in your slide. Carefully focus in this region to determine if it is visible. As eggs are produced by the ovary, each is fertilized by sperm coming in from the seminal receptacle, yolk is added around the fertilized egg from the vitelline glands and a hard, thin shell is put around all of it by the shell gland. As the eggs proceed down the uterus, the shells harden and assume their final shape and texture. Most fluke species have different shaped ootypes (the chamber where the eggs are enclosed by the shell) and uteruses, and hence each species has a unique egg shape. With a little practice it is quite easy to recognize a species by only seeing its eggs!

As part of this work, decide which surface of the fluke is uppermost on the slide (use fine focus). With this knowledge, determine whether the testes lie above of below the intestines.

In making your drawing, make appropriate breaks to show what organs lie above or below others.

3. Class: Cestoda

Tapeworms are very highly specialized flatworms adapted primarily for life in the intestines of other animals. Taenia is a representative genus although there are many which parasitize humans. The reproductive systems are expanded, while the digestive system is entirely lost. How could such an animal "eat" or acquire its nutrition if it lacks a mouth, stomach, intestine, and anus? Obtain a prepared slide of a tapeworm. These slides contain sections of a tape worm taken from different locations.

The "youngest" (= newest) body units, proglottids, are produced near the scolex and are quite small and, internally, undifferentiated at first. As they are pushed to the rear by continual new growth, they become larger, broader and packed with eggs (gravid). In the prepared slides, locate the scolex and the immature proglottids next to it. Find the mature proglottids and finally the gravid proglottids. Do the proglottids change shape as they mature? _________________

Describe the shape of the immature, the mature and the gravid proglottids below:

Immature: ______________________________________________________

Mature:_________________________________________________________

Gravid:__________________________________________________________

The scolex is provided with, in this species, four suckers which are arranged circularly around it. Does the rostellum of the scolex have hooks? _________ Some species lack hooks, some species lack a rostellum. Draw the entire scolex of this specimen. We will not examine the internal anatomy of this class in detail.

II. PSEUDOCOELOMATES

Read the introduction to the pseudocoelomate phyla on p. 302 of your text; refer particularly to Figure l6-1 for different body plan cross sections. A glance at the illustrations in chapter 16 will indicate a wide range of structural diversity of the groups called "pseudocoelomates". These phyla are probably not closely related but originated independently quite a long time ago from various ancestors.

A. Phylum Nematoda (round worms)

Roundworms are an extremely large and diverse group of organisms. They are of major health importance as many are parasites of humans and their domestic animals. In addition, many soil nematodes attack the roots of commercially important crops, stunting or often killing the plants. Not all nematodes are parasitic, but the ones we will consider today are major human parasites which are capable of causing death in heavy infections.

We will examine one of the largest known species of nematode, the intestinal roundworm Ascaris, which occurs in the intestines of humans and pigs. Preserved specimens of both sexes are available; males are the shorter specimens with the distinctly "hooked tail".

Obtain a prepared slide showing Ascaris in cross-section. The slide will usually (but not always) contain sections of both sexes. Look first at the larger female. The cuticle is very distinct, stained a light purple color. You will also get a good idea of the body musculature, which looks very diffused and spongy. What might this indicate about the animal's life style?

The two large rounded or oval structures full of small spheres are the two uteri full of eggs, which have already had sperm penetration in most cases. Using high power on a compound microscope, you can see the contents to some extent, and you may be able to see some dark staining chromosomes of the female pronucleus as it undergoes meiosis.

The intestine is usually flattened or otherwise distorted but is easy to identify as it is composed of a single layer of long columnar epithelial cells (see Fig. 20-7, p. 439 and fig 6-4, p.143). Notice where the nucleus of the cell is located. Is it centrally located? _____ Generally, secretory cells have their nuclei on the opposite margin of the cell.

Although the worms are protected on the outside from the host's enzymes by the thick cuticle, whenever they feed on the intestinal contents they must be ingesting the same kind of digestive enzymes. What do you think protects the cells of the worm's mouth and digestive system from being digested by the host's enzymes?

Occasionally in these cross-sections one can locate the dorsal and ventral nerves and the two lateral chords, but not always so don't spend too much time looking for them. The other, smaller round objects with a central oval lumen are the ovaries seen in cross-section.

Look at the large cavity in the worm in which the reproductive and digestive organs are suspended. This cavity is the pseudocoelom and is formed by the retention of the old embryonic blastocoel. The blastocoel was derived before the mesoderm was formed and hence the mesoderm does not completely line the adult pseudocoel. The longitudinal muscles are mesodermal in origin. Can you find them? Locate the intestine. Is it surrounded by a coating layer of thick muscles? In a pseudocoelous animal the mesodermal layer does not contact the endodermal layer, and, therefore, there is no mesodermal tissue to support the intestine. The pseudocoel in the living animal is fluid filled and in addition to floating the internal organs it functions as a hydrostatic skeleton making the worm rather turgid and stiff.

The cross-section of the male is basically similar in terms of body wall, muscles, intestines, etc. to the female. There will be two large, round to oval bodies which seem to be rather densely granular in texture and stained a kind of purple color. This is a section through the seminal vesicle which stores sperm prior to copulation. There will be a number of smaller, round objects with fairly large round inclusions of a dark purple color; these are various loops and folds of the long, single tubular testes.

B. OTHER NEMATODES

Trichinella spiralis:

Trichinella spiralis (the trichina worm) causes the disease trichinosis. It is a parasitic roundworm which, as a larva, encysts in mammalian muscle tissue. Obtain a slide of Trichinella and examine it under low power. You will see the encysted worms in oval shaped capsules (cysts) throughout the muscle tissue. Adults live in the intestine where copulation occurs. After mating the males die and the females retain the eggs, giving birth to living larvae (up to 1,500 per female). The larvae are released to the intestine where they penetrate into the intestinal wall and enter the blood stream. They are swept throughout the body and lodge in most organs and tissues. In muscle tissue, however, the larvae become encapsulated and may survive for upwards of 30 years. When meat is eaten that contains the encapsulated larvae, the stomach acids digest off the capsule wall, which is composed of calcium carbonate, and the larvae are released into the new host's intestinal tract. In about 2 days the worms have matured to adult forms.

Ancylostoma sp.

The genus Ancylostoma along with the genus Necator comprise the nematodes known as hookworms. Obtain a prepared slide of a hookworm. This is an adult worm which has been removed from the intestinal mucosa of the host. Notice that the head end is flexed down or slightly curved toward the ventral surface. This curvature of the head gives the parasite its common name of hookworm. The anterior end also has a widely gaping mouth that has modified cuticular plates or teeth. These are difficult to see in side view, but you should be able to carefully focus on at least one tooth at a time. Be careful not to break the slide as you focus. Now look at the posterior end of the hookworm. In the female the posterior end tapers to a point. In the male, the posterior end flares into a wide copulatory bursa. What is the sex of your specimen?

After copulation, eggs are released by the female directly into the intestine of the host and they mix with the feces. If the feces are scattered on the ground, the eggs hatch into a larval form which lives in the soil. Contact of bare skin with the soil (as in walking around barefoot) will result in the larvae penetrating through the skin and into the blood stream. Once in the blood stream, the young worms are swept to the lung capillaries and they break through to the alveolar sacs. They crawl up the trachea to the pharynx where they are swallowed and wind up in the intestine.

Phylum Rotifera.

Living specimens of Rotifers may be available. If so, make a slide and observe these microscopic filter feeders. They may be gliding through the water, or attached by hooks at the posterior end to pieces of debris on the slide. Rotifers get their name from the ciliated crown that surrounds the mouth and serves to direct food particles to the mouth opening. If you look closely, you will see a structure that looks like a beating heart just below the mouth. This is a feeding structure called the mastax. Its movement grinds the food particles before they reach the gut.