Classification of Mollusks
By SeashellWorld.com
All mollusks have soft bodies. In fact, Mollis is Latin and Malakos is Greek for soft. The study of mollusks is known as Malacology. Most mollusks develop hard shells to protect them from their hostile environment. Their fleshy mantle secretes a shell in generally a single or pair of valves. Based on their shell type, scientists and educators engaged in Malacology have classified mollusks into six classes:
Gastropods:
These creatures, which are also known as univalves, have a single, shell that is usually coiled. They have a distinct head with tentacles and a rasping tongue (radula). Most of the 105,000 known species in this group have shells. Half of the species are marine while the remainder is either terrestrial or fresh water. The sexes are separate in most marine species. Free-swimming larva emerges from eggs laid into the water. This larva eventually grows into a shell adult. The average life expectancy of a gastropod is five years while some have survived to 30 years. They may be carnivorous, herbivorous or parasitic.
Bivalves:
Also known as pelecypods, they are mollusks with two valves joined by a hinge. Most of the 20,000 species are marine including clams, mussels, oysters and scallops. About one third of the species live in fresh water. They lack a head and radial teeth. They feed on microscopic plant life aided by their gills. The tow shells (valves) are kept closed by strong muscles. The sexes may be combined in one individual or may be separate. Most lay their eggs directly into the water. Oysters, clams, and scallops are a major source of food.
Cephalopods:
This group of 600 species includes squid, octopus and nautilus. These very active animals have large eyes, powerful jaws and anywhere from 8-90 tentacles.
Tusk Shells - Scaphopods:
These curved, tooth-like shells number about 300 marine species. Also known as scaphopods, they are common to shallow and deep water. They lack a head, eyes, or true tentacles.
Chitons - Polyplacophors:
These primitive, flattened, marine mollusks have 8-plated shells imbedded in tough tissue and held in place by a muscular ring called the girdle. They live on rocks near shore and feed on algae. There are about 600 shallow water species.
Monoplacophora:
A very rare, deep-sea primitive group that until recently, was only known from fossils. Today, there are six known living species.
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