Mollusks

The Phylum Mollusca is second in size only to the Phylum Arthropoda. Although members of this phylum are extremely diverse in structure, physiology, and habits, they are built on a uniform body plan consisting of a mantle, shell, and foot. The mantle secretes the shell and is sometimes involved in respiration. The shell provides support, protection, and secure attachments for muscles. The foot is fleshy and contains most of the viscera.

Figure1. Representative of the Phylum Mollusca. (A) A clam, Class Bivalvia; (B) A snail, Class Gastropoda; (C) A tooth shell, Class Scaphopoda; (D) An octopus, Class Cephalopoda; and (E) a chiton, Class Polyplacophora.

Class Gastropoda. This is the largest and most widely-distributed of the molluscan classes. The name "gastropoda" literally means "stomach-foot" (these animals crawl around on their stomach). Examples of gastropods include snails, abalone, limpets, nudibranchs, and slugs. While the snails, abalone, and limpets have retained their shells, those of nudibranchs and slugs have been secondarily lost. Snails typically have spiral shells, sometimes with an operculum (a "trap door" used to close off the entrance of the shell when the snail withdraws to the interior). Abalone shells superficially resemble a single clamshell. Their shell, however, has a row of respiratory holes along one edge. Limpet shells are conical with a hole at the top (also an adaptation for respiration). Nudibranchs are shell-less marine forms with exposed gills (their name means "naked gill"); some are extremely colorful. Slugs, for our purposes, are snails without shells. Gastropods are found on land and in both fresh and salt-water habitats. Depending on the species, they may be predacious, herbivorous, or scavengers. A few are adapted for suspension feeding. Gastropods, like other mollusks, have a peculiar file-like tongue called a radula.

(PIX 1; PIX 2)

Class Polyplacophora (= Amphineura). Polyplacophorans (chitons) have shells composed of eight overlapping plates that at least partially covered by the mantle. They are almost prehistoric in appearance. Most feed on algae and other encrusting organisms by scraping them off the surface of rocks with a radula. They are common inhabitants of the intertidal zone.

Class Scaphopoda. Scaphopods are a small group of mollusks commonly known as "tooth shells" or "tusk shells". They are rarely seen alive since they're relatively small and burrow in the sediment. Microorganisms make up the bulk of their diet. Adhesive tentacles sometimes capture larger food. Tooth shells are all marine. Examine the scaphopod shells on display.
Class Cephalopoda. Octopods, squid, cuttlefish, and nautili are examples of cephalopods (the term "Cephalopoda" literally means "head-foot"). Most of the adaptations seen in the cephalopods are related to their swimming and predatory behavior. The mouth is surrounded by a circlet of tentacles (or arms). The shell is usually internalized (squid pens and the cuttlebone of cuttlefish) or absent (Octopus). Only the nautili and fossil forms retain a shell. That of the nautili is chambered and used both for protection and control of buoyancy (compare a sectioned nautilus shell to a gastropod shell, if available). The shells of the now-extinct ammonoids were extremely thick with complex sutures (some fossilized specimens may be on display). Cephalopods are voracious carnivores. Their eyes are among the best of the invertebrates and eight or more arms can quickly encircle prey. A parrot-like beak with poison glands subdues and tears prey. Clams, snails, crustaceans, and/or fish may be eaten, depending on the species. Cephalopods are restricted to marine waters.
Class Bivalvia. Members of this class include clams, mussels, scallops, and oysters (this class is also known as Class Pelecypoda or Lamellibranchia).

Figure 2. Structures associated with feeding in the clam.

Figure 3. Internal structure of a clam

Mollusc radulas	Mantle edge : showing the sensory, secretory and muscular lobes and the three layers of the shell: periostracum, prismatic layer and nacreous layer	Internal Structures of a Mussel
Clam Gill	Clam Section showing mantle, shell, and foot	A more dorsal section showing the ctenidia, foot, digestive gland, and haemocoel.