Phylum Mollusca has both aquatic and terrestrial species, and includes clams, snails, octopuses, and sea slugs. Mollusks are characterized by their soft bodies, which are usually protected by a hard calcium carbonate shell. However, this shell can be highly reduced or completely absent in some representatives of the phylum.
For example, the gastropods (meaning "stomach foot") include snails, slugs, sea slugs, and nudibranchs. There are terrestrial, marine and freshwater species in this group. Most snails have a coiled shell, however, the shell is completely absent in nudibranchs. Terrestrial gastropods (e.g., snails) lack the gills that are characteristic of other mollusks.
Bivalves (“two shells”) are another groups of mollusks that includes mussels, oysters, and clams. They have a reduced head, two hinged shells connected by strong muscles, and a body that is highly modified to fit within these shells.
The cephalopods (“head foot”) include octopuses, squid, cuttlefish, and chambered nautiluses. The members of this group have a head surrounded by tentacles, which can be used for locomotion and grasping prey. The shells (also called pens) of squid and cuttlefish are reduced and internal. Recent research suggests that cephalopods are intelligent organisms, and British institutional research review boards have gone so far as to enact special ethical protections for squid and octopuses similar to those afforded to higher level vertebrates.
The largest living invertebrates (animals without backbones) are the giant squids, which are extremely elusive and typically only encountered washed-up on the shore. The largest, documented giant squid was 18 meters long. The individual in this photo is 9 feet long.
The internal anatomy of a representative mollusk (a snail) reveals how much more complex these organisms are than representatives from any of the preceding animal phyla discussed
In general, the mollusk body plan consists of a muscular foot, a visceral mass, and a mantle. The foot is used for movement (especially in the gastropods) or as an anchor. The visceral mass houses most of the internal organs (e.g., the stomach, gonads, and heart). The mantle is the tissue layer that covers the visceral mass. In organisms that have shells, the mantle produces the shell. Underneath the mantle is a mouth at one end and a mantle cavity at the other. The mantle cavity houses the anus and gills.
Mollusks characteristically have a radula (a rasping structure that is used to scrape food particles from hard surfaces). For example, the radula of mollusks cleans algae off of the surfaces in aquariums. Some snails use their radula to drill into the shells of other mollusks.
Visit the marine aquarium in the HUB on the University Park campus and look for the radula on the underside of the snails cleaning the walls of the tank.