Phylum Mollusca
General Characteristics:
  • Bilateral Symmetry with segmentation reduced.
  • Body covered with a thin membrane called the mantle. This structure may secrete the shell.
  • Three body parts: head, muscular foot, and the visceral hump ( contains the internal organs).
  • Gas exchange occurs through the gills or the lining of the mantle cavity.
  • Nitrogenous wastes are excreted through nephridia.
  • Open circulatory system. Does not contain many blood vessels.
  • Reproduction sexual with the development of the trochophore larvae.
  • There are 4 classes of mollusks: Amphineura, Bivalvia, Gastropoda, and Cephalopoda.
  • True coelomates. Its body cavity is completely lined with mesoderm.
  • Protostomes. The mouth develops first in the embryo.
Class Amphineura: Marine organisms containing a shell of 8 plates. They contain a muscular foot for locomotion. Their head is reduced. There are about 650 species of these organisms containing the Chiton. They cling to rocks in the inter tidal zone and use a tooth like radula to scrape food off of the rocks. They breathe through gills.
Class Gastropoda: This is the largest molluscan class. Snails and slugs are the most familiar forms of this class. They are characterized by a single shell. This shell has a characteristic cone shape. The oldest part of the shell is its tip. They breathe through gills or the mantle cavity acts as a primitive lung. Their muscular foot extends along the ventral surface of the organism. Their head contain a pair of antennae and a pair of eyes. They contain a scrapping radula to pick up food from the environment. The slug does not contain a shell and its usually becomes active during the cool moist part of the day.

Class Bivalvia: This class contains animals covered with 2 shells. The clam and the oysters are the most evident examples of this class. These organisms are mainly marine living in the inter tidal zone . The body has a large visceral hump and a muscular foot with a reduced head. The Visceral hump and foot are covered with the thin mantle. The mantle produces a cavity where the gills and 2 siphons ( the ecxurrent and incurrent siphons) are found. The animals are filter feeders. They take water containing food into the mantle cavity and pass it over the mouth. The excess water is forced out through the excurrent siphon. The shell contains 3 layers: the outer horny layer, inner prismatic layer, and the inner pearly layer. Pearls are sometimes produced when an irritation enters the mantle cavity and covered by the pearly layer secreted by the mantle. The oldest part of the shell is the area surrounding the hinge, called the umbo.

Class Cephalopoda: Squids, octopuses, cuttlefish, and Nautiluses are examples of this class. All members of this class are marine. Squid and octopuses are among some of the most intelligent of the invertebrates. All cephalopods are carnivorous. They contain tentacles and a central mouth containing a sharp beak. Many of these organisms contain a large ink sac that is used when trying to escape from potential predators. They have well developed eyes and brains. They contain special pigment cells called chromatophores which enable them to change color. They have gills in the mantle cavity. Their circulatory system is a closed one.