Final answer:
Mollusks generally have a head with sensory tentacles, a muscular foot for locomotion, a mantle that can secrete a shell, and a radula for feeding. They have a coelom, complete digestive system, and excretory system. The three main classes of mollusks are gastropods, bivalves, and cephalopods, the latter of which sometimes lack external shells.
Step-by-step explanation:
The body of a mollusk typically includes several key features: a head with eyes or tentacles, a muscular foot for movement, a mantle which secretes the shell in many species, and a feeding organ known as the radula. The mantle also forms the mantle cavity, which is distinct from the coelomic cavity and is involved in gas exchange.
Mollusks are characterized by a complete digestive system, a coelom, and an excretory system that includes nephridia. Furthermore, mollusks possess gills or use their mantle cavity for gas exchange, depending on whether they are aquatic or terrestrial.
The three major classes of mollusks include gastropods, bivalves, and cephalopods. Among these, cephalopods such as squids and octopuses do not have external shells. The molluscan body plan varies among classes, with some adaptations including a reduced shell or a different use of the muscular foot.