The Phylum Mollusca
The mollusks make up one of the largest phyla of animals, both in numbers of living species (at least 47,000) and in numbers of individuals.
A significant characteristic of mollusks is their possession of a fluid-filled cavity that develops within the skin. The coelom not only functions as a skeleton but also provides space within which the internal organs can be held.
All mollusks have a soft body which is generally protected by a hard shell. In some forms however, the shell has been lost in the course of evolution, as in slugs and octopi or greatly reduced in size and internalized, as in squids. Squids now have what is similar to a skeleton in their bodies.
Mollusks are also characterized by a toothed tongue, the radula, composed primarily of barbs. The radula serves both to scrape off algae and other food materials and also to convey them backward to the digestive system. In some species, it is also used in combat.
A significant characteristic of mollusks is their possession of a fluid-filled cavity that develops within the skin. The coelom not only functions as a skeleton but also provides space within which the internal organs can be held.
All mollusks have a soft body which is generally protected by a hard shell. In some forms however, the shell has been lost in the course of evolution, as in slugs and octopi or greatly reduced in size and internalized, as in squids. Squids now have what is similar to a skeleton in their bodies.
Mollusks are also characterized by a toothed tongue, the radula, composed primarily of barbs. The radula serves both to scrape off algae and other food materials and also to convey them backward to the digestive system. In some species, it is also used in combat.