Explain the Phylum Molluska - Mollusks?

You are probably very familiar with members of this phylum. The mollusks include the octopuses, squids, cuttlefish, scallops, oysters, clams, mussels, slugs, and snails. Even though they widely differ from each other in their overall appearance, upon close inspection, the bodies of all of these animals have several major features in common. All mollusk bodies consist of three main portions: a visceral mass of internal organs; a muscular foot for locomotion; and a mantle, which covers the visceral mass and secretes a shell.

The visceral mass includes the organs that carry out the functions of digestion, reproduction, circulation, and excretion. The mantle is a fold of tissue covering the visceral mass, and produces a water-filled mantle cavity, which bathes the gills. The gills not only exchange gases with the water that surrounds them, but they also help in the feeding process by filtering and funneling food toward the mouth with mucus. Clams, mussels, scallops and oysters are the famous filter feeders of the ocean. The foot in many species is used to help the mollusk move about and to burrow, as in the bivalves, but it may also be adapted to help attach the mollusk to the substrate. The foot in squids and octopuses is highly modified into what we call tentacles, and they are used to capture and kill prey.

Another distinguishing feature that all mollusks (besides bivalves) have is a feeding structure called a radula. A radula is basically a toothed rasping file that is used by herbivorous species to scrape food such as algae off rocks. Vegetable gardeners and farmers are certainly very familiar with the radulae of snails and slugs! Carnivorous mollusks use their radulae to drill through the hard shells of their prey. The radula of the poisonous cone shell has evolved over the years into a poisonous dart, which is used to kill prey. These cone shells are deadly, and, in an instant, have killed humans unfortunate enough to step on them in the ocean.

We will discuss three of the most familiar Classes of Mollusks: Class Gastropoda-the snails and slugs, Class Pelecypoda-the bivalves, and Class Cephalopoda-the squids and octopuses