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Biology 18 Spring, 2008
Lab 5: Phylum Mollusca
Objectives: Understand the taxonomic relationships and major features of mollusks Learn the external and internal anatomy of the clam and squid Understand the major advantages and limitations of the exoskeletons of mollusks in relation to the hydrostatic skeletons of worms and the endoskeletons of vertebrates, which you will examine later in the semester
Textbook Reading: pp. 700-702, 1016, 1020 & 1021 (Figure 47.22), 943-944, 978-979, 1046
Introduction The phylum Mollusca consists of over 100,000 marine, freshwater, and terrestrial species. Most are familiar to you as food sources: oysters, clams, scallops, and yes, snails, squid and octopods. Some also serve as intermediate hosts for parasitic trematodes, and others (e.g., snails) can be major agricultural pests.
Mollusks have many features in common with annelids and arthropods, such as bilateral symmetry, triploblasty, ventral nerve cords, and a coelom. Unlike annelids, mollusks (with one major exception) do not possess a closed circulatory system, but rather have an open circulatory system consisting of a heart and a few vessels that pump blood into coelomic cavities and sinuses (collectively termed the hemocoel ). Other distinguishing features of mollusks are:
z A large, muscular foot variously modified for locomotion, digging, attachment, and prey capture. z A mantle , a highly modified epidermis that covers and protects the soft body. In most species, the mantle also secretes a shell of calcium carbonate. z A visceral mass housing the internal organs. z A mantle cavity , the space between the mantle and viscera. Gills , when present, are suspended within this cavity. z A radula , a protrusible, rasp-like feeding organ present in most, but not all, species. In herbivorous mollusks (e.g., chitons and snails), the radula is used for scraping algae from rocks. In carnivores, the radula can be fang-like and is used for piercing prey (e.g., squids and octopods), or may be pointed and used for drilling through shells (e.g., some snails).
Of the five classes of mollusks, four (listed below) are fairly common, and the first three will be studied in the laboratory (Figure 1): z Class Bivalvia , clams, scallops, and oysters; characterized by a hinged shell of two valves (parts) and a foot used for digging; lack a radula; marine and freshwater filter feeders. z Class Gastropoda , snails, slugs, whelks, limpets, abalones, and nudibranchs; usually possess helical shells and a foot used for crawling; marine, freshwater, and terrestrial herbivores and carnivores. z Class Cephalopoda , squids, octopods, and nautiloids; usually lack external shells; possess a siphon for jet-propulsion; marine carnivores. z Class Polyplacophora , the chitons, primarily herbivorous marine species with a shell consisting of many plates (hence its name).
You will be given unpreserved organisms to dissect, and there will also be mounted slides for you to examine under the microscope. This year, you will also have living clams to work with. Careful dissection may enable you to see muscular contractions of the large foot and maybe even of the small heart! By working with living organisms, we hope that you will gain a greater appreciation for the dynamic aspects of animal organ systems.
Figure 1. A generalized mollusc (center) and its relationship to the principal molluscan classes (from Sherman & Sherman, 1976, The Invertebrates: Function and Form , 2 nd^ ed.).
except for the outer edge around the margin of the shell where glands lay down shell material. As a result of the lack of cilia, any particles that become lodged between the mantle and the shell in this area cannot be removed - instead, the particles are covered with nacre, or inner shell material (and become what bits of wisdom?).
- The mantle encloses the soft visceral mass (yummy!). Inside are housed the gonads and much of the digestive system (more yummy!). The muscular foot (even more yummy!), which can be engorged with blood, lies ventral to the visceral mass. What types of structures allow the foot to retract and extend?
Figure 2A & 2B. Mercenaria. See legend on next page. (from Sherman & Sherman, 1976, The Invertebrates: Function and Form , 2 nd^ ed.)
dorsal
ventral
(gills)
Figure 12. 2
Left valve
Right valve
the clam is usually considered to be "open" (what does this mean?). Blood is pumped out of the heart through several arteries into a series of large sinuses (parts of the hemocoel). Blood from the sinuses flows into the gills, and then back into the heart.
B) Class Gastropoda (snails, slugs, and nudibranchs), preserved specimens
- Examine the slide of a snail radula. What is the function of this structure?
- Examine the external morphology of Busycon , a large, marine snail better known as a conch ( Figure 3 ). Pick up the coiled shell and note the large opening that allows protrusion of the foot and head. How must the body be modified to fit within such a shell? Where do you think the posterior end (anus) of the animal is located? Snails occur in both marine and freshwater habitats, and some are even found on land. In terrestrial gastropods, gills are absent, but the mantle cavity has become highly vascularized and serves as a lung. Such snails are referred to as "pulmonate".
Figure 3 : Important features of the marine conch ( Busycon spp) (from Barnes 1980, Invertebrate Zoology)
C) Class Cephalopoda, fresh-frozen squid
Part A: External Anatomy.
Examine a fresh-frozen squid, which was thawed out earlier today. Unlike other mollusks, the shell of squids is not external but rather is internal (and much reduced in size). A tough, muscularized mantle completely surrounds the animal ( Figure 4 ).
- The head of the squid should have eight arms of about the same size, known as grasping arms. It should also have two longer arm-like structures called tentacles. What typical structure of mollusks are the arms and tentacles derived from?
- How do the tentacles differ from the grasping arms? Use scissors to remove one of the tentacles and observe its end using the dissecting microscope. What do the specialized ends of the tentacles look like?
- The mouth, encircled by the tentacles and grasping arms, is equipped with a radula that has been modified into a hawk-like beak. Note the large eyes, which function much the same way as the eyes of vertebrates
- The term cephalopod means “head-foot.” Why do you think squid got this name?
- Rinse your squid under running water before beginning your dissection. Hold the squid vertically in the stream of water with the tentacles pointing upward so that water flows into the mantle cavity. Tilt the head back from the siphon and stand back! What happens?
How does the action of the siphon enable the squid to rapidly propel itself through water?
- Observe the squid’s skin and look for spotted areas. These spots contain cells that have color-producing pigments (called chromatophores ), which allow the squid to change its color and pattern. Pull off a section of this thin layer of skin and observe on the dissecting microscope. Why might it benefit the squid to rapidly change its appearance?
- What internal structure do you expect to see in the squid that functions in defense?
- Given the above morphological and anatomical features, what sort of lifestyle do you think the squid leads? Would you expect to find them in the same habitats as snails or clams, and if not, where would you expect to find them?
Figure 4. Squid, continued
The Squid Nervous System
Brains : Where is the brain in the squid? How large is it relative to the size of the body? How does this size ratio compare to that of the earthworm? What does this tell you about the amount of central processing that the brain of each animal performs?
Invertebrate Vision Systems: One striking similarity that you will observe later in the semester in the visual systems of vertebrates is in the way their eyes are structured. Because vertebrates are relatively closely- related organisms, it is not unusual that they have eyes with very similar structures. We call these types of structures homologous structures, because their structural and functional relatedness is based on the common history of the organisms being compared. What is really amazing is that even relatively unrelated organisms, such as cephalopods and mammals, can have very similar eye structures. We call this type of similarity convergent evolution.
In today’s lab, make sure you examine the squid eye (Figure 5). Use the comparative eye diagram (Figure 6) to note your observations, as well as to compare the eye structures of organisms you will not dissect in lab (marine sandworm, spider).
Figure 5 : Lateral view of eye and optic lobe (brain) of the squid, Todarodes , showing position of the optic gland (from Baumann, 1970, The extra-ocular light receptors of the squids, Brain Res.).
Figure 6 : Comparative eye anatomy with major features indicated by number, with labels below. Use this diagram to take notes on for the squid eyes, noting things like color and indicating what features you could see in the dissection and which you couldn't (from Carolina Biological).
