Phylum Molluska: Mollusks

The mollusks constitute one of the largest phyla of animals, both in numbers of species and in number of individuals. They are characterized by soft bodies within a hard, calcium-containing shell, although in some forms the shell has been lost in the course of evolution, as in slugs and octopuses, or greatly reduced in size and internalized, as in squids. There are three major classes of mollusks: (1) the gastropods, such as the snails, whose shells are generally in one piece; (2) the bivalves, including the claims, oysters, and mussels, which have two shells joined by a hinge ligament; and (3) the cephalopods, the most active and most intelligent of the mollusks, including the cuttlefish, squids and octopuses.

Although the mollusks diverse in size and shape, they all have the same fundamental body plan. Their body is basically bilaterally symmetrical, and they have a true coelom. There are three distinct body zones: a head-foot, which contains both the sensory and the motor organs; a visceral mass, which contains the organs of digestion excretion, and reproduction; and a mantle, which hangs over and enfolds the visceral mass and which secretes the shell. The mantle cavity, a space between the mantle and the visceral mass, houses the gills, and the digestive, excretory, and reproductive systems discharge into it. Water sweeps into the mantle cavity propelled by cilia on the gills, passing through the gills and aerating them. It then passes by the nephridia, gonophores and rectum, which are always downstream from the gills. Water leaving the mantle cavity carries excreta and, in season, gametes.

The digestive tract is far more convoluted and so provides more convolution and more working surface than that of the annelids. In all mollusks, the digestive tract is extremely ciliated, with many different working areas. Food particles are absorbed by the cells lining the stomach and the anterior intestine, and, from them, it is passed into the bloodstream.

Mollusks get food in different ways. A bivalve filters food particles from the water that flows under its mantle. Gastropods or univalves, eat with a scraper which is like a tongue with teeth. This toothed organ called radula - a characteristic organ of this phylum. It serves both to tear or scrape off algae and other food material and also to convey them backward to the digestive tract. Octopuses and squids use sucking discs on their powerful arms to seize food.

Octopuses and squids have a good way to escape from enemies in a hurry. They use their mantles to squirt water in a forward direction. This can propel the organism backwards. In the meantime, the octopus or squid can squirt out a dark ink that helps into hide and confuse their enemies.

Mollusks have gills. A gill is a structure with an increased amount of surface area, through which gases can diffuse, and enrich body supply for the transport of these gases.

Mollusks have three-chambered hearts; two of the chambers (atria) collect oxygenated blood from the gills and the third (the ventricle) pumps it to the oxygen-depleted tissue. Cephalopods, which are extremely fast and active animals, have accessory hearts that propel blood into the gills.

Many mollusks pass through a similar larval stage of development. The mollusk larva, called a trochophore, is pear-shaped with a band of cilia around its middle. Because a trochophore swims about freely, it helps disperse the species. This opportunity is especially important for mollusks, which are encumbered with heavy shells as adults. Some marine annelids also pass through a trochophore larval stage. This similarity has led scientists to conclude that annelids and mollusks are closely related groups.

Mollusks reproduce sexually. The man clam, for instance, releases sperm into the water. The sperm are swept to the eggs under the female’s mantle.

Mollusks are classified according to the kind of shell they have. Of the seven classes in the phylum Molluska, the three major classes are two-shaped mollusks, such as clams, scallops and oisters; one-shelled mollusks, such as snails; and head-footed mollusks, such as octopuses, squids, and cuttlefish.