INVERTEBRATE ZOOLOGY 03050 WEEK 8 SUMMARY: THE MOLLUSCS 10/18/99
Text (7th ed.): Ch. 21. Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)
possible web site: http://is.dal.ca/~ceph/TCP/index.html (The Cephalopod Page)
1. Phylum Mollusca ("soft"): A large, successful phylum of 100,000 + species that containing animals as different as snails (Class Gastropoda), clams (Class Bivalvia), and squids (Class Cephalopoda). To understand this diversity, we will highlight basic features and compare how each of the different classes modify the basic body plan.1,2
2. Model of a mollusc (mollusk) with 3 distinct regions: 1) the head-foot with most of the nerves and sense organs, a radula (rasp-tongue) and all of the movement (locomotion) organs; 2) the visceral mass containing the digestive, reproductive, and excretory organs; and 3) the mantle covering over the visceral mass and secreting a shell (i.e., the mantle is always below the shell). (Your text talks about only the head-foot vs. the visceral mass, but I think the mantle can be large and distinctive enough to highlight as well). Also, the space between the visceral mass and mantle/shell is called the mantle cavity, which is a space for respiratory functions, as well as a discharge region for the digestive, excretory, and reproductive systems. Inside this mantle cavity are found a pair of the characteristic gill of the molluscs, the ctenidium (-a, pl.). The ctenidia consist of sets of filaments covered with cilia (microscopic hair-like processes, certainly useful in the filter feeding of the bivalves). The flow of blood in the gills is dorsal to ventral, while the water flow is ventral-dorsal (counterflow).2 The water flow is created by the cilia on the gills. The gills separate the mantle cavity into a ventral inhalant portion and a dorsal exhalant portion.
3. Differences among body regions. The head-foot functions largely by muscles which have fast reflexes (like bilaterally symmetrical animals) and the visceral mass and mantle cavity function slowly and continuously using mucus and cilia. Think of this model mollusc as two very different units: a muscular unit responsible for locomotion and a ciliary unit responsible for breathing, eating, etc.2 In different classes, which unit predominates (or as I suggest above, how does the mantle also get modified)?
4. Movement in molluscs. Body fluids in molluscs are almost all blood, and internal body sinuses are almost all for blood, supplied by sets of pumping hearts within their own pericardial cavity.2 The circulatory system of molluscs (except cephalopods) is open in that it is not contained within blood vessels like our closed circulatory system. In molluscs, blood flows in sinuses around internal organs. All this blood serves as 1) O2 carrier, and 2) a hydraulic system for transmitting forces by distant muscle contractions. In other words, while you are looking at the live molluscs in lab, you should note that the soft extendible parts like tentacles, siphons, etc. can be rapidly contracted by muscles, but only slowly extended by blood pressure from blood coming in from another part of the mollusc.
5. General life cycle. Usually separate sexes and external fertilization. Several larval stage (free-swimming trochophores, veligers in marine; a"hitchhiking" veliger or glochidium in freshwater bivalves) before much shell formation from the mantle.
6. Class Gastropoda ("belly foot"): the snails. The largest and most varied mollusc class, with a coiled shell (except slugs). To accomodate living in a shell, snails undergo torsion during development. This is a 180o counterclockwise twisting of the visceral mass, the mantle, and the mantle cavity in relation to the foot, so that living in a shell works out better. Note: torsion has nothing to do with the shell coiling! Furthermore, snails develop additional internal asymmetry (loss of many right-side structures) to live in a coiled shell and prevent being fouled by mantle cavity wastes. Snails move by using a flattened foot and a slime trail, i.e., the head-foot predominates over the visceral mass/mantle. Gastropods can have gills and/or lungs (modified mantle cavity).
7. Class Bivalvia ("two valves"): the clams and oysters. These animals are covered by a sheet-like mantle and a shell of two valves (hence name), so that the head-foot is less important. Most are filter feeders. The shell appears like that of two parts in adults, although develops as one unit in the embryo.
The oldest part of the shell is the umbo, which is dorsal and (at least points) anterior. This means morphologically that the"edge" of the shell is ventral, where simple eyes often found (e.g., blue eyes in the mantle of a scallop1), and that the shells represent a right and a left. Adductor muscles are located at either end of the dorsal half of the shell to keep it shut. As filter feeders, bivalves do not have a head or radula, and have greatly expanded gills and use of the mantle cavity. The mantle can secrete nacre around sand grains or other irritants inside to form pearls.
In the freshwater USA, bivalves demonstrate environmental issues. The US is/was home to many species of freshwater mussels and clams, but which are going extinct at a high rate (biodiversity losses are not only tropical!). Furthermore, the introduction of zebra mussels into the Great Lakes and other freshwater bodies has been"exciting," as they clog up water intake valves, etc., and crowd out native species.1
8. Class Cephalopoda ("head foot"): the squids and octopuses. In many ways, these are the most advanced of all the invertebrates we will study this semester. The anterior portion of their foot has been modified into tentacles or arms. The posterior part of the foot has also been modified into a funnel associated with the mantle cavity for a jetlike propulsion (especially in squids). The mantle makes up large portion of animal (e.g., as in the lateral fins of the squid). In contrast to other molluscs, cephalopods have a closed circulatory system for their higher energy demands.1 Fossil cephalopods were mostly shelled, more like we see in the nautilus today. In most living cephalopods, the shell is modified to a pen in squids or absent in octopuses. Powerful beak-like jaws and raduli are used to tear apart food that is spotted by a very human-like eye. The brain is highly developed in the cephalopods and octopi have even shown learning. Cephalopods have pigment cells or chromatophores that can change color rapidly, and also contain an ink gland behind the anus that is also effective in defense.
9. Minor classes of molluscs: Class Polyplacophora ("many plate bearing"): the chitons. With a large sucker foot and an extended mantle margin protruding from eight overlapping plates of a shell, chitons cling to shore rocks to scrape up algae.
10. Class Scaphopoda ("boat foot"), the tusk shells (the shells used in wampum). They are one to two inches long and burrow head first into the sea bottom. The tapered end of the shell projects above the sand surface and water for respiration is pumped in and out of the opening. They feed on small organisms caught by the head tentacles.
11. Class Monoplacophora ("one plate bearing") and evolutionary relationships: Neopilina, a living fossil. Thought to be extinct for 300 million years, it was dredged up in 1952. It has a cap-shaped shell with a broad, flat foot. Internally, it has serially repeated gills and foot retractor muscles, which has led to excitement regarding the relationships of the molluscs, annelids, and arthropods. This pattern in Neopilina is not the strict repetition of mesoderm and coelom that we will see next week in the earthworm.3
12. Comparison of molluscs to arthropods. Having an incomplete exoskeleton in the form of the shell may influence the form and function of the molluscs, but not to the same extent that the arthropod external skeleton does for them--compare squids to snails, for instance, vs. comparing a scorpion to an insect. The presence of specialized structures such as a radula and ctenidium in all of the mollusc classes, though, argues for a unified although very diverse group.3
1 CP Hickman et al., Biology of Animals, 7th ed. (WCB McGraw-Hill, Boston, 1998), pp. 451, 461, 464.
2 WD Russell-Hunter, A Life of Invertebrates (Macmillan, New York, 1979), pp. 349, 351, 352, 356.
P Willmer, Invertebrate Relationships: Patterns in Animal Evolution (Cambridge U. Press, NY, 1990), pp. 47-48, 252, 268
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This page updated on October 25, 1999