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Zoology 216 Exercise 6 -
Mollusca & Annelida Return
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Phylum: Mollusca |
Phylum Annelida |
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In lab today, we will examine two closely related Phyla, the Mollusca and the Annelida. Both are eucoelomates but their coelomic cavities are quite different. In molluscs, it is a small cavity around the heart, while in annelids it is a large, fluid filled space which also serves as a hydrostatic skeleton. Annelids are considered to be the first truly metameric (segmented) phylum. While most biologists agree that the molluscs are unsegmented, there has been some controversy in this regard (see text for discussion). There is still some debate as to whether they evolved from a segmented ancestor, along with the annelids, and lost their segmentation, or if segmentation developed in annelids after the two groups diverged.
I. Mollusca:
It seems difficult to imagine that the molluscs which we will examine in this lab could be classified into a single phylum, or even be considered descendants of a common ancestor. Each class has developed remarkable modifications associated with widely different feeding strategies, which in turn have led to chain-reactions affecting other organ systems. There is not time in one lab period to examine molluscan diversity in any detail; we can only hope to get an idea of several extreme examples. See figures 21-35, 21-36 and Classification of Molluscs, p. 472 for a summary of the molluscan phyla.
Of the eight presently recognized classes of mollusks, only five are available to us. Only the last two, Cephalopoda and Bivalvia, will be examined for internal structure. It is generally agreed that mollusks and annelids had a common ancestor, on the evidence of chiefly embryonic development (both schizocoelomate, protostomate and probably metameric). Some authorities prefer to think that the two groups diverged prior to the onset of segmentation, a point which cannot be settled at present. Only in the two classes Monoplacophora and Polyplacophora is there indication of metamerism in the basic bilateral symmetry, paired muscles, paired gills, paired kidneys and gonads. Most authors, however, still regard this as an unsegmented phylum. Consult text and lecture notes for details.
A. Class: Polyplacophora
Chitons are common marine animals with no freshwater representatives. They are usually associated with rocky seacoasts where species attain lengths of twelve inches or more. There are a number of specimens on demonstration. Note the general body form. Why would this body form be advantageous in the crashing surf?
Note the large muscular foot on the ventral surface. Anteriorly you can see next to the foot a smaller, circular fleshy mass which has an opening in the center. This is the buccal mass with the mouth. Can you see any tentacles or eyes or other head appendages?
B. Class Gastropoda:
It is thought that all ancestral gastropods were shelled, and that present day shell-less species (terrestrial "slugs" and marine "nudibranchs") have subsequently lost the shell. Gastropods obtain oxygen either through gills (external or internal) or through a form of lung (vascularized mantle cavity). Gastropods feed with the aid of a remarkable device called a radula, a long flexible rasping organ which is covered with small teeth. It is used to scrape algae and other small organisms from the surface of rocks or to tear the flesh from victims.
Obtain a dish containing one or more aquatic gastropods and examine it under low power of a dissecting microscope. When the animal moves, you can notice the gliding motion of the foot region, reminiscent of that of flatworms. Above the front end of the foot is a separate head region bearing two extensible tentacles. In many gastropods, there is an eye at the tip of each tentacle; in others the eyes are located near the base of the tentacles where they appear as tiny black spots. Where are the eyes of this specimen located?
In some freshwater species you will see a thin round sclerite just behind the shell as the snail moves. This operculum will close over the aperture of the shell when the snail pulls inside, affording it protection from predators and desiccation. The specimens we have in lab today do not have an operculum.
Examine specimens as they crawl over inverted microscope slides so you can observe the action of the radula and the foot. As the snail crawls along, the mouth opens rhythmically and the radula comes down to the surface against which it moves with a forward and upward motion.
C. Class Cephalopoda
Squids are predators feeding mainly on fish. They are swift and skillful swimmers and are mainly active at night. Their body structures show many adaptations to this rapidly moving lifestyle. Examine a specimen which is on demonstration.
Orientation: In cephalopods, as the name implies, the head and the foot regions have become coalesced and subdivided into numerous arms (8 in octopods) and tentacles (which are longer and have suckers only near the tip). How many of each does the squid have? Arms: ______ Tentacles: _____
The body orientation may seem strange at first but remember that the head and foot are on the same surface. The arms and the tentacles are anatomically ventral, so that the dorsal side is the opposite end (the pointed end away from the tentacles). Functionally, the squid swims backwards with the dorsal end going first
Notice how streamlined the animal is; the head-foot mass seeming to emerge from a tubular body. Near the posterior end is a broad pair of lateral fins which function like elevators on an airplane's wing and tail surfaces. The main body mass is composed of the tubular mantle, very tough and muscular. On one side (we will call this dorsal) the mantle projects forward over the foot and on the other side (ventral) it is a little indented. Dorsally there may be a thin, brown horn-like object sticking out. This is called the gladius or "pen". Ventrally, there is a large triangular median structure with an opening on its anterior end. This is the funnel, used in locomotion.
A squid can swim along entirely by the movement of its two large fins, or it can supplement this by taking water into the mantle cavity, which can then be tightly closed down in front, and thrusting the water out with great force through the funnel opening. The funnel can be directed in any position resulting in the squid rapidly darting forward, backward or to either side by a kind of jet propulsion. By the combined use of the fins and funnel, a squid can change direction of movement almost instantly. Using the mantle-funnel technique, speeds of up to 45 mph can be briefly attained, and squids can break the surface to escape predators and "fly" for quite some distance.
Examine the suckers. Note that each is a cup-like object on a narrow flexible stem. The inside of the cup is kept circular by a chitinous lining with the outer edge being toothed. Inside each is a fleshy core (piston), which, when the cup is seated against the surface, can be retracted to create a small suction. The teeth keep the cup in place. These make wonderful devices with which to hold a slippery fish. In giant squids (up to 20 m in length) each sucker may be as big as a dinner plate. What do such creatures feed on? On each side of the head you can see the site of the eyes, which are usually poorly preserved. They are built very much like the eyes of vertebrates, an astonishing case of convergent evolution.
D. Class Bivalvia (formerly Pelecypoda)
All bivalves have two opposed shells (valves) although in some species they may be very small (see Fig. 17-27). Since all depend upon filter feeding for nutrients, the basic body form is similar among most species. A few species (e.g. scallops) live on the ocean floor and can move freely by "clapping" their valves to produce a jet-propulsion effect, but most are sedentary and rely on their ability to draw in a current of water which is filtered by the gills. Many remain buried in sand, mud or gravel (some inside solid rock in chambers rasped out by their shells) and extend a part of their body (the siphon) to obtain water.
The two valves are not top and bottom (dorsal and ventral) but are right and left valves. The side where the valves are hinged is dorsal. Here there will be a swelling on each valve called the umbo. The edge of the clam from which the foot is protruded (or would in a living specimen) is the ventral surface. A swollen place on the dorsal side of each valve, the umbo, is closest to the anterior end of the clam and tends to hook slightly toward the anterior. The siphons would protrude from the posterior end. To determine which is the right and which is the left valve, hold the clam with the dorsal side (umbos) toward you, with the anterior end up and the posterior down. The animal's right valve is now on its (and your) right side.
Marine mussels produce free swimming veliger larva (see Fig 21-6), but freshwater mussels have an unusual life cycle. Sperm are released into the water, and siphoned in by the female mussel where they fertilize eggs held in the mussels's gills. These fertilized eggs develop into parasitic larvae called glochidia (singular=glochidium; see Fig 21- 29). These glochidia are released into the water where they attach to a host fish (or in one case, a salamander) where they become encysted and undergo metamorphosis into a young mussel. This may take one to two weeks after which they drop off and develop into sedentary clams. Can you see any advantage to this parasitic stage in fresh water species?
E. Class Scaphopoda
All tusk shell are marine. The specimens on display belong to the genus Dentalium, which has a very wide distribution. Locally, we also have another genus on our Atlantic beaches, Cadulus. Look carefully at Dentalium. Notice how the sides of the shell taper to a smaller opening. In Cadulus, the 2 openings are smaller in diameter than the center of the shell. Cadulus thus appears to have a bulge in the middle. It is a small scaphopod, measuring about 1/2 inch long. Both genera of scaphopods were prized by native American Indians who used them for decorations, jewelry, and money. It is interesting to note that both Dentalium and Cadulus show up in a lot of indigenous Indians' clothes from the Great Plains. Extensive trade routes between the coastal Indians and the various Indian nations of the central part of the continent must have existed.
II. Annelida:
The animals we examined last week, the pseudocoelomates, represent a distinct advance over flatworms and cnidarians in terms of functional morphology. For instance, they have developed a complete digestive tract and a body cavity in which the internal organs are largely independent of movements of the body wall. The pseudocoelom is nothing more than the embryonic blastocoel which is retained into adulthood.
The Annelids, like the molluscs are eucoelomate animals. Before proceeding with today's lab, you should have read over the material on pages 475-476. Notice in particular, from Fig. 22-1, how much more complex the body structure of an annelid is by comparison with that of a roundworm. The main advantages possessed by segmented worms are: l) a far greater efficiency in locomotion, owing to better local control of short longitudinal and circular muscles, and 2) possession of numerous fluid-filled body units (segments) useful in circulation and reproduction. In this phylum, the three classes show an interesting progression from obviously generalized polychaetes, with their typical lateral parapodia, into more specialized oligochaetes which lack parapodia (except for some vestiges), and finally into highly specialized leeches.
A. Class: Polychaeta
Nereis (=Neanthes) virens. Nereis is an excellent example of a generalized, free-living, carnivorous marine polychaete. Obtain a good specimen, rinse off and blot dry with a paper towel. This is a good example of a metamerically segmented body in which all of the segments are similar except the anterior-most two. Locate the structures labeled in Fig. 22-2A, this is a dorsal view which shows the pharynx everted. In preserved specimens it is usually retracted. Compare yours with the drawing. Is the pharynx of your specimen everted or inverted (retracted)? If inverted, the jaws have been pulled inside, but you may be able to feel them with a probe or needle.
Other Polychaetes: Several other kinds of polychaetes are on demonstration, giving a partial idea of different body forms in this group. Arenicola is a burrowing form (Fig. 22-10), in which the parapodia have been secondarily reduced. Aphrodita is a crawling surface form like Nereis, but with a totally different body shape. Often Aphrodita is called the "sea mouse." (This is a good example of a "tongue-in-cheek" name since Aphrodite was the Greek goddess of love and beauty. A more charitable interpretation is that like Aphrodite, who was born of the sea foam and cast on the beach, the sea mouse too can be found cast upon the shore following storms). There are many types of additional polychaetes, some free-living, some commensals (scale worm), and some tube dwellers (Amphitrite, Sabella, Chaetopterus). Many are small with body lengths less than 10 mm. (See reference photos of living specimens: figures 22-3 - 22-5)
B. Class: Oligochaeta
Lumbricus (or related genus). Obtain a specimen, rinse and blot dry. Externally you can see the obvious segmentation and the saddle-like or band-like clitellum (Fig. 22-11B) which is present only in sexually mature adults. By running your finger along the ventral side you can feel the tips of the bristles (or setae) which are the same as the parapodial acicula seen in Nereis. On segment 15 (the prostomium is not counted) there is a pair of conspicuous transverse openings on the ventral side. These are the male gonopores or openings of the vasa deferentia (more on this later). The mouth is underneath the prostomium. The peristomium is not enlarged and specialized as in Nereis (why not?), and on the dorsal side it is completely separated by a median lobe of the prostomium.
Holding the specimen dorsal side up, make a shallow cut starting at the 5th segment. This cut should not go deeper than the body wall to avoid severing the dorsal blood vessel, so work under a microscope and use gentle sawing motions with the blade held flat against the body. Note: if you sever the dorsal blood vessel, the blood will run out of the closed circulatory system making the hearts more difficult to find. Continue this cut all the way back to the clitellum. You will also need to extend the cut foreward to the first segment in order to observe the brain. Now put away the blade and use a toothpick or pin to break the intersegmental septa which are fairly prominent in earthworms. Work mainly on one side only until you can spread the body open and see the internal organs. This will get easier as the specimen slightly dries (but moisten the specimen if any parts start to get hard and brownish).
The most conspicuous organ in the anterior region is the large muscular pharynx inside segments 4-6 (or 4-7). It cannot be everted as in Nereis but does have a lot of external muscles attached to it, especially to the inside of segment 4. The pharynx merges directly into the esophagus which runs back into the large, dark-colored crop in segments 15-18. It will be hard to see because it is overlaid with large reproductive organs and five pairs of "hearts" all of which are squeezed into segments 7- 17. Just posterior to the crop is the equally large but lighter colored gizzard which is a very muscular homogenizing and grinding mill. You might want to make a partial cross-section of the gizzard to see how thick the muscle is. Posterior to the gizzard the simple stomach-intestine runs back through the body to the terminal anus.
The major blood vessels are a dorsal and ventral artery. The dorsal vessel can usually be seen where it crosses over the crop and gizzard, and further forward over the top of the esophagus. In segments 7-11 one can find the enlarged, reddish-brown lateral connectives ("hearts" or aortic arches) which connect the dorsal and ventral vessels. The septa in preserved worms are stronger in this region and require determined picking to separate. You may be able to follow one or more of the hearts down to the ventral vessel.
The dominant reproductive organs are the three pairs of large whitish seminal vesicles in segments 10-12. The two pairs of testes are small and located under them in segments 10 and 11 and are difficult to locate. However, in segments 9 and 10 the two round whitish seminal receptacles are readily visible. Sperm are released into the testis sacs in segments 10-11 and picked up by a pair of funnels which lead back into the vas deferens on each side; this exits on segment 15. Start at segment 15 and search for this thin, pale tube and with luck you can trace it forward to where it disappears under the seminal vesicles. The several female structures, ovaries and ovisacs, are very hard to find and should be passed over today. Page 483 and Fig. 22-14 describe and show the curious way in which earthworms manage mutual insemination, and should be consulted at this point.
Lastly, the nervous system consists of a large, solid, mid-ventral nerve cord and circumpharyngeal connectives. Going back to the front end of your worm, where the dorsal cut was started at segment 5, try slipping the end of a toothpick forward of this cut (over the top of the pharynx) and then extending the incision forward (the toothpick should prevent cutting too deeply), as far as the prostomium. Careful spreading of the cut edges here should show the little transverse white "brain" in segment 4 just at the front end of the pharynx. By pushing the latter to one side and tearing some muscle with a pin tip, you might be able to trace one of the connectives down to the first big ganglion on the ventral nerve cord.
Examine a prepared slide of a cross-section of the earthworm. To aid in identification of structures, refer to Fig. 22-11C. Locate the following structures: Intestine (with lumen), dorsal and ventral blood vessels, ventral nerve cord, circular and longitudinal muscles, cuticle.
C: Class Hirudinea
Leeches seem pretty clearly to have been derived from some oligochaete ancestor, and there are several small enigmatic groups which have been placed first in one class, then in the other. There are no traces of setae remaining in the leeches, and the septa have largely disappeared, producing a large continuous coelom. But leeches do develop a clitellum during the mating season, and lay their eggs in cocoons just as oligochaetes
Some leeches are on demonstration. Note the shape of the body. Are they long and cylindrical like Nereis or Lumbricus ? Can you find their mouth? Do they have "jaws" or teeth? How do they attach to their host? Look at the posterior end with its suction cup. Is it smooth inside or are there attachment hooks? How do you think a living leech moves, attaches, &c.? Look at the anterior end
of a large leech. Does it have eyes? If so, how many. Using a dissecting microscope, scan back along the edges of the leech. Do you see any other structures? If so, what might be their function? See page 485 of your text and check out Haementeria. How much blood do you think it could store from a single meal?