The Mollusks Help
Introduction to The Mollusks
Closely related to the segmented worms is another group of coelomates, the mollusks ( MAHL -usks). This phylum of mollusks consists of a huge number of more than 100,000 different species of invertebrates with “soft bodies” ( mollusc ) that are nonsegmented. Mollusks include clams, snails, oysters, squids, and octopuses. “What could such more-or-less round-shaped organisms without segments possibly have in common with the segmented annelid worms?” you might well ask. The answer is that both annelids and mollusks probably have a common ancestor in the Fossil Record, and they were the first two groups of animals to develop a true fluid-filled coelom cavity, lined with mesoderm cells.
Basic Body Plan Of The Mollusks
Because they have a soft body, many kinds of mollusks are protected by a rock-hard, calcium-rich shell. Those early mollusks who developed this adaptation would obviously have enjoyed a much greater protection from hungry predators. Squids and octopuses have either a much-reduced shell, which is mostly internal, or they have lost their shells entirely.
Despite their obvious differences in degree of protection by shells, the mollusks all share a common basic body plan. The basic plan is shown in Figure 10.5. The three main parts of any mollusk are the mantle , visceral ( VIH -sir-al) mass , and foot . The mantle is a “little cloak” or covering on the back of the mollusk. The covering tissue within the mantle produces the hard shell in many species. And since it is on the back or superior surface of the mollusk, the mantle functions in respiration, exchanging gases with the surrounding air or water. In hundreds of mollusk species, there is a mantle cavity also present below the mantle itself. The mantle cavity contains, in turn, a set of gills . Since the mantle is hardened by an additional shell, it has a reduced surface area available for diffusion of oxygen (O 2 ) into the blood, and diffusion of carbon dioxide (CO 2 ) out. This reduced surface area available for respiration is compensated for by the gills, which assist with this critical job.
The visceral mass is the main soft body of the mollusk, which contains the major viscera ( VIH -ser- ah ) or “guts” (internal organs). The coelom cavity is also found within the visceral mass. It holds the heart of the organism. The foot of the mollusk is the inferior portion of the visceral mass, which, somewhat like a human foot, is large and fleshy and flat. The foot pushes against the ground or sea bottom, propelling the mollusk forward.
In addition to their basic body plan, many species of mollusks are classified as bivalves ( BUY -valves). This is because they have “two” ( bi ) shells hinged tightly together, like a “valve.” This double-valve, when opened, gives the organisms bilateral symmetry. Bivalves include such well-known invertebrates as the clams, oysters, mussels, and scallops.
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