Lower Invertebrate Groups.  Worms.

Platyhelminths - Planaria.

Animals of the Phylum Platyhelminthes are flatworms - in fact, that's what the name means in Latin. They are the most primitive group to have a mesoderm, or middle layer of cells, which is also why they are the most primitive group to have organs and organ systems. The organism we will look at in this part is a fairly typical free-living Platyhelminth, the planaria. Get one from the specimen bottle by "blowing" it off the bottom with a blast from the dropper, then picking it up and quickly transferring it with the dropper to a slide. Keep the drop fairly small, so the worm won't have too far to roam, but be sure that you don't let it dry out.

Note that the worm always moves in a predictable direction, leading with one end - the evolution to place many sense receptors and their processors at this leading, head end is called cephalization; behind the head, mirror-image sides have evolved as well, a layout called bilateral symmetry. The head end sense receptors that you can see include EYESPOTS, non-focussing light detectors, and AURICLES, the side "flaps" that are used to sniff the water with chemical receptors.

1. How do the auricles move as the animal moves around?

Add some water so the animal has room to move and react. This next part may work better if you set the slide on a white background and observe by eye. You are going to use a probe to touch the animal and record its reactions. Don't jab it, just touch.

2. Describe the reaction when the planaria is touched on the back end.

3. Describe the reaction when the planaria is touched on the front end.

4. Describe the reaction when the planaria is touched on the side .

Return the animal to the specimen bottle and get the slide marked "Planaria Digestive System." Look at it under the microscope at low power. This specimen has been chemically treated to make it more transparent, then stained to make the gut show up. The rectangular structure in the middle of the worm is the PHARYNX; the MOUTH is at the posterior (rear) end of the pharynx, and the anterior (front) end of the pharynx opens into the INTESTINE, which branches three ways, like a "Y." The Y shape would be more obvious if the intestine didn't have lots of side branches called DIVERTICULA.

5. What is the main purpose of these diverticula?

Make a sketch of your preserved specimen in the space below. Label ALL structures discussed above (the ones in all capital letters) as well as the AURICLES and EYESPOTS.

Platyhelminths -  Clonorchis.

Clonorchis sinensis is a flatworm parasite (they live on and harm other animals, called hosts) known as the Chinese liver fluke. Their life cycle, after hatching from eggs, involves a snail intermediate host (that's where a parasite reproduces asexually), then such carrier hosts as crustaceans and fish, leading to a final or determinate host (where it reproduces sexually) of humans. Clonorchis is from a platyhelminth subgroup, the digenetic (two lives or two beginnings) trematodes, a group that includes many other causes of human disease.

Make a sketch of your preserved specimen. You are going to label it with ALL structures listed here in all-capital letters. It is extremely important to first figure out which end is which - reading though the description before starting your labeling should help.

The anterior (head) end of the worm has a ORAL SUCKER, like a thick doughnut but on its side. About one-third of the body length posterior is another, face-up "doughnut," the VENTRAL SUCKER. Just posterior to the oral sucker is the PHARYNX, a muscular pumping organ that pulls food into the wishbone-shaped DIGESTIVE TRACT. At the posterior end of the body is a small EXCRETORY PORE; anterior to that are two fairly large, branched TESTES. Continuing anterior, you'll see a SEMINAL RECEPTACLE and an OVARY, which leads to the UTERUS, a structure that winds back and forth in the worm and is full of EGGS. Out on the edges are dot-like VITELLARIA, which produce yolk for the eggs (you may see ducts leading to the ovary from them). Near the ventral sucker is the SEMINAL VESICLE, a CIRRUS SAC and a CIRRUS (penis), but often the sucker is on one or more of these structures, hiding them.


Platyhelminths - Taenia.

Taenia is a tapeworm, a member of the Cestode subgroup of Platyhelminths. The adults live inside the intestines of vertebrates, attached to the side by a specialized head, the scolex, and generating segments, proglottids, that mature and reproduce. These worms can be quite long - whale tapeworms can be a hundred feet long.

1. Different species of tapeworms are, as adults, very different characteristic lengths. What other feature should very long species have in common?

Get a microscope slide that says Taenia scolex. This is the attachment end of the worm.. These worms, as larvae, get into intermediate hosts and reproduce - when the final host eats the intermediate host, the worm larvae attach and become adults in the small intestine. Sketch the scolex in the space below and label: the HOOKS that are used to dig into the intestinal lining; they're arranged in a ring around the domelike ROSTELLUM; the SUCKERS (there are four, but usually only two are visible); and the budding PROGLOTTIDS at the other end, where the worm has been cut.

Mature Proglottids. The segments of the tapeworm, as they get older and further from the scolex, become sexually mature, developing male and female systems. Eventually they mate, with proglottids of other worms or other proglottids from the same worm, produce eggs, and break off. The host passes either whole proglottids or eggs in its feces. Get a slide labeled "Taenia - Mature Proglottid." In the space below, sketch one proglottid (there will be a few on the slide - you can combine features from different proglottids to produce your drawing) and label with the following all-capital structures. The posterior end (on the right or left) is wider than the anterior end. One side (they're supposed to alternate from one proglottid to the next) has a bump with an opening, the GENITAL PORE; just inside the pore is the CIRRUS in the CIRRUS SAC. A SPERM DUCT runs to the cirrus from many dot-like TESTES found mostly in the anterior half of the proglottid. At the posterior end is the slightly flattened, triangular VITELLARIUM, the yolk gland; anterior to that are two round, paired OVARIES. You might be able to see, between the ovaries, a starry-shaped MEHLIS' GLAND or a small round SEMINAL RECEPTACLE. From this area, a tube, the UTERUS, extends anterior and branches toward the genital pore - it will eventually fill the proglottid with eggs. Running along the sides and across the anterior ends of the proglottids are EXCRETORY CANALS and NERVE CORDS.


Nematodes - Cephalobus.

Nematodes are generally known as roundworms, since their bodies are cylindrical. They have a protective outer layer called a cuticle and efficient internal organs in an open body cavity filled with high-pressure fluid. Our specimens are of the Genus Cephalobus, found in just about any kind of decaying plant matter - a lab culture can be made just by burying a piece of potato for a couple of days and then transferring it to the lab (the white material they're in is potato). There are probably more than a million species of Nematodes, and they can be found virtually everywhere (there are species that live in wine, or pure vinegar!), from free environments to parasitic lifestyles on animals and plants. In fact, there's about a 75% chance that you right now are harboring at least one species.

Put a small drop of water on a slide. Put a bit of material from the bottom of the Cephalobus culture tube in the drop. Put a cover slip on the drop and look at it under the microscope.

1. Describe what you see.

2. How do the individuals differ from each other? Why are they different?

3. Find a large specimen that's barely moving and sketch it. The outside is its CUTICLE; it has a MOUTH at its anterior end and an ANUS at its posterior end; the INTERNAL ORGANS include digestive and reproductive systems.



The Annelids are commonly called the segmented worms, because their bodies are made up of a series of separated chambers, or segments. Get a specimen from the jar and observe it. Draw a sketch and label from the following description. There's a specialized head segment, the STOMODEUM, and a specialized tail segment, the PROCTODEUM. The stomodeum may have ANTENNAE and MOUTHPARTS visible, depending upon the species of worm. Between the stomodeum and proctodeum, the SEGMENTS are very similar to one another. Each segment has a pair of bristles, called SETAE, to help it move.

2. Describe how your specimen moves.

Molluscs. Snails.

The Molluscs are a group of muscular, complex animals that often produce simple shells; they include snails and slug, clams and their relatives, and octopus and squids. As adults, these animals bear little resemblance to one another. We're going to look at snails early enough in their lives to see similarities that disappear later.

1. If they are available, observe baby snails through a dissecting microscope. Compare it to a planaria flatworm - how are they similar, how different in appearance and movement?

2. If they are available, observe snail embryos in eggs under the microscope. Draw a sketch below and label the EYES and HEAD. Can you see some resemblance to an octopus here?



First Written 1990;  Last Update 2001;  Web Version 2001,  M. McDarby




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