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Anatomy of Animals


Mollusks

Coelom All the advanced invertebrate phyla have a true coelom. Nevertheless, they can be divided into two groups on the basis of embryological evidence (Figure 09). In mollusks, annelids, and arthropods, the coelom forms by splitting of the mesoderm. Therefore, they are called the schizocoelomates. In echinoderms and chordates, the coelom forms by outpocketing of the primitive gut. It is thus called enterocoelomates. Note the interchange of mouth and anus in these two different types of development.

Figure 09 Formation of Coelom [view large image]

    Mollusks are a very large and diversified group containing many thousands of living and extinct forms. However, all forms of mollusks have a body composed of three distinct parts:

  1. Visceral mass: the soft-bodied portion that contains internal organs.
  2. Foot: the strong, muscular portion used for locomotion.
  3. Mantle: the membranous or sometimes muscular covering that envelops but does not completely enclose the visceral mass. The mantle cavity is the space between the two folds of the mantle. The mantle may secrete a shell.
In addition to these three parts, many mollusks show cephalization and have a head region with eyes and other sense organs.

The division of the body into distinct areas seems to have allowed diversification to occur because there are many different types of mollusks adapted to various ways of life. The molluskan groups can be distinguished by the modification of their foot. In bivalves, such as clams, the foot is laterally compressed and is sometimes called pelecypods ("hatchet-footed"). In the gastropods ("belly-footed"), such as snail, the foot is ventrally flattened, and the animal moves by muscle contractions that pass along the foot. In cephalopods ("head-footed"), such as octopuses, the foot has evolved into tentacles about the head. The tentacles are used to seize prey.

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