The Mollusca are true coeloms with a specialized foot. They have a fold in the dorsal wall that is referred to as the mantle. This mantle encoloses the mantle cavity and usually contains gills and secretes the exoskeleton. The rasping organ is called a radula except in bivalves. There are 90,000 species and Mollusca is second only to arthropods in the number of named species. The significant classes are:
* Bivalvia/Pelycypoda – This class contains clams, mussels, and scallops * These are found in freshwater and marine habitats. They are sedentary filter feeders with a wedge-shaped foot, dorsal hinge, while cephalization is reduced. * They are the second largest Mollusca class with 20,000 species * Gastropoda – Largest class with 70,000 species
* Includes slugs, snails, conch and whelk
* Marked by an asymmetrical coiled shell with a well developed head. * The radula and foot are large and flat,
* Cephalopoda – This includes: nautiloids, squids, and octopus. * Contain both a head and foot.
* The shell is almost completely reduced or absent.
* The foot is modified into arms or tentacles.
Looking at the dissection pictures, one can see that they have a well developed circulatory system to aid them with living in the ocean. Photo below shows cephalopod circulatory system. Image taken from biology.ualberta.ca
Moving on to the digestive system, one can see that it is specialized for what a cephalopod eats. Initially there are jaws for crushing and biting that lead into an esophagus and then a stomach. There is also an ink gland and digestive gland that aid in defense and digestion of food. Image taken from biology.ualberta.ca.
From the image of different Mollusca shells, it can be seen that there is a wide variety of shells in the phylum. Seeing as how there are many different species, a diversity of shells is not surprising. Image taken from biology.ualberta.ca....