Section 1

Mollusca

• Mollusks and annelids have true coeloms. Most aquatic mollusks and annelids develop from a trochophore.

• A mollusk's body is divided into the head-foot and the visceral mass, which contains the internal organs. Most mollusks have at least one shell, which is secreted by a layer of epidermis called the mantle. Aquatic mollusks have gills through which gas exchange takes place.

• The main feeding adaptation of most mollusks is the radula, a tonguelike structure that is modified in different species for scraping, drilling, or harpooning.

• Gastropods undergo torsion—the visceral mass twists during larval development. Snails and most other gastropods have a single shell, while some gastropods, such as slugs and nudibranchs, lack shells. Gastropods move by means of wavelike, muscular contractions of the

• Bivalves have a shell that is divided into two valves, which can be pulled together by contracting powerful adductor muscles. Bivalves lack a distinct head and have no radula. Most are sessile and filter food from the water. In clams, water enters through an incurrent siphon and exits through an excurrent siphon. Food is strained from the water as it passes through the gills.

• Cephalopods, including octopuses and squids, are free-swimming, predatory mollusks with numerous tentacles. They have an advanced nervous system with a large brain and well-developed sensory organs. Cephalopods have a closed circulatory system and do not pass through a trochophore stage during development.

Vocabulary trochophore (p. 705) visceral mass (p. 706) mantle (p. 706) mantle cavity (p. 706)

ganglion (p. 706) radula (p. 706) gastropod (p. 707) hemolymph (p. 707)

hemocoel (p. 707) bivalve (p. 708) incurrent siphon (p. 709) excurrent siphon (p. 709)

cephalopod (p. 710)

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