The Main Characteristics of How Mollusks Are Classified
Mollusks account for one of the largest phyla of animals with more than 50,000 species. This diverse group of animals includes saltwater, freshwater and terrestrial invertebrates. Some familiar species include snails, slugs, clams, oysters, mussels, scallops, squids and octopuses. Mollusks are classified into six classes, and several of their diverse characteristics dictate how these animals are grouped.
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Mollusk Classes
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The phylum Mollusca is divided into six classes, the most commonly recognized being gastropods, bivalves and cephalopods. Each class contains species with widely varied characteristics. Generally, gastropods include snails and slugs, bivalves encompass shellfish and cephalopods are marine animals like octopuses and squids. The other classes are Amphineura, Monoplacophora and Scaphopoda.
Shell Presence and Type
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This first distinguishing feature of mollusks is the presence or absence of a shell, and the type of shell. Shellfish like clams and oysters clearly have shells, as do terrestrial animals like snails. Most gastropods have a single shell, like the spiral shell of snails and conchs. Members of the gastropod subclass Opisthobranchia have no shells. Bivalves have a hinged, double shell, like that of mussels and oysters. Of the cephalopods, including octopuses and squids, only the nautilus has an external shell. Scaphopods, or tusk shells, have single cone-shaped shells. Chitons that make up the Amphineura subclass Polyplacophora, have an armor of overlapping plates. The Amphineura subclass Monoplacophora consists of wormlike sea creatures with small spines on their bodies but no shells.
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Foot Presence and Purpose
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Another characteristic of most mollusks is the presence of a foot, a muscular appendage that aids in locomotion. Gastropods, meaning "stomach foot," have a foot. This appendage is what allows a slug to crawl by gyrating its muscular underbelly. Chitons are sometimes thought to be sedentary, but they actually crawl at an extremely slow pace, sometimes moving only 10 feet in one year. Bivalves have feet used for burrowing. Cephalopod feet are actually arms or tentacles used to propel the animal through the water.
Radula Function
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Most mollusks have a radula: a toothed "tongue" that extends from the body to capture food or scrape algae from rocks and the seafloor. Some species have hundreds or thousands of radulae with small, sharp teeth. Some species' teeth contain poison and are used for defense and aggression. Gastropods and cephalopods have some form of radula, while bivalves have none.
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References
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