ZOO 137 - Invertebrate Zoology Phylum Info File -- Mollusca

Kingdom Animalia - Subkingdom Metazoa
Phylum Mollusca (etymology: ______________________________________________)

Class Polyplacophora (etymology: ____________________________________)
(examples: ________________________________________________)

Class Gastropoda (etymology: _______________________________________)
(examples: ________________________________________________)

Class Bivalvia (etymology: __________________________________________)
(examples: ________________________________________________)

Class Cephalopoda (etymology: ______________________________________)
(examples: ________________________________________________)

Things You Should Do Before Coming to the Lab on Mollusca

o Memorize the classification given above.
o Study Chapter15 in Animals Without Backbones.
o Complete exercises 51-56 in The Zoology Coloring Book (optional but highly recommended.
Check the highlight sheets in lab for parts you do not need to color).
o Check to see that your dissecting kit is in your backpack.

Things You Should Do During the Lab

There will be but one "regular" lab devoted to observing specimens of this, the second largest phylum of animals. A second lab period will be devoted to a special seminar starring the cephalopods - a really neat group of invertebrates for a number of reasons.

o Dissect a fresh water mussel (or clam, depending on available specimens) using PAZ Figs. 7.6 -7.8 as your guide (ZCB plates 53-55 will also be helpful). After you have checked out the outer surfaces of the shells (valves), open them to expose the body. To do this may require you to cut one or both adductor muscles; ask for assistance in doing this. Know all the structures in the PAZ figures except the following: 7.6 (no exceptions); 7.7 (nos. 1-2, 7, 9); 7.8 (nos. 3-4, 6, 12, 13) (Figs. 7.9 and 7.10 are of a razor clam; ignore them). Most of the features you need to know are external. To open the visceral mass, use your scalpel to make a sagittal cut beginning with the ventral edge of the foot, slicing with a sideways motion as you would use a knife to open a hamburger bun. Your slice will reveal the only internal structures you need to recognize on a dissected specimen: the intestine, the gonad, and the digestive gland. Note that PAZ Fig. 7.11 is referable to the model you will see and that there are structures depicted here you can't see well in the dissection but need to know from the model (see below).

o Examine the model of the clam. For the cross section portion of the model, you need to know all the structures depicted in PAZ Fig. 7.11 except mantle artery and periostracum. Note also that Fig. 7.11 lacks a guideline connecting "hinge" to the appropriate structure on the drawing (the dorsal reddish area); add one. The portion of the model illustrating a lateral view of a whole specimen (which valve is missing?) is directly referable to Figs. 7.7 and 7.8.

o Examine the external structure of a squid. To complete this will require that you cut open the mantle (if it's not already opened) along its posterior surface to view the visceral mass and gills (ctenidia). Know all the features listed for PAZ Fig. 7.15, to which you should add visceral mass, stellate ganglia and pen. The pen you can observe by gently pushing the visceral mass to one side. Take care to note the orientation of the body of the squid compared to that of the mussel/clam. The squid's body is greatly elongated along its dorsal-ventral axis, and very shortened along the antero-posterior one. Instead of dorsal and ventral surfaces, it has a dorsal and ventral ends! Instead of anterior and posterior ends, it has anterior and posterior surfaces!! As a result of all this reorientation, the mouth (surrounded by the arms; spread them out so you can see it; the beak may also be peeking out) and anus (opening into the siphon, or funnel) have come to lie very close to each other.

o Examine the model of the internal anatomy of a snail. There is little you need to know about the structures illustrated by the model, but it does illustrate the results of the curious phenomenon called torsion, a permanent twisting of the body that happens during development. Check out ZCB Plate 32.

o Examine the following demonstration specimens and slides:

-- A prepared slide showing a snail radula. PAZ Fig. 7.3 is a greatly magnified view of the radula.

-- A prepared slide of glochidia (sing. glochidium); these are the parasitic young of certain freshjwater clams.

-- The buccal bulb of a squid (dissected out of the head from its normal position at the base of the arms. Squids and other cephalopods have powerful beaks like that of a parrot that they use in conjuction with the radula to shred their prey. The bulb is a sherical, muscular structure; you can see the beak at one end and esophagous at the other. Creepy, huh?

-- Specimens illustrating the four principal mollusc classes (see classification above): chitons, octopus, squid and the shells and preserved specimens of various bivalves and gastropods as well as the nautilus. You need to know the common and class names for each one.

Things You Should Do After the Lab

o Using all sources available to you, make a list of ten important features, characteristics, etc. of the molluscs -- don't overlook the fundamental "parts list" we discuss in lecture.

o Be sure to correlate what you've observed in lab with your readings and lecture notes.