MOLLUSCA

MOLLUSKS

    Molluscs include the familiar snails, garden slugs, and sea slugs (class Gastropoda), the clams, oysters, scallops, and mussels (class Bivalvia), the active octopuses, squid, cuttlefish and Nautilus (class Cephalopoda) as well as some less familiar groups such as the classes Aplacophora (shell-less worm looking things called solenogasters), Caudofoveata (small shell-less deep-sea worm like things), Monoplacophora (living fossils - only 8 or so species), Polyplacophora (chitons) and Scaphopoda (tusk shells). In terms of numbers of species, Mollusca is the second largest phylum of invertebrates (Arthropoda is the largest). Mollusks comprise a group of soft-bodied animals. The most common characteristic of mollusks is their shell.  Snails are univalves, which means they have one shell.  The bivalves, or two-shell mollusks include the clams, scallops, and oysters.  Still other mollusks have lost their shells altogether. The octopus, the squid, and the sea slugs have evolved their own survival strategies to replace their protective armor. Indeed, it is due to the absence of a protective shell the octopus has evolved the largest and most complex brain of all the mollusks.  Mollusks are one of the most important  phylums of animals for the fishing industry.

Slug

Squid

Precursor to Human Systems: Digestive tract with 2 openings

TYPES OF MOLLUSKS

Gastropod: (also called univalves) are a type of mollusks that have a single valve, a shell, which is sometimes reduced or even absent, and a muscular foot. There are over 90,000 species of gastropods worldwide, both in the water and on land. Some gastropods include snails, whelks, and slugs.

Banana Slug

Bivalve: Bivalves (pelecypoda) are soft-bodied animals that are protected by two hard shells, hinged together.  The approximately 15,000 species of this type are found in marine and freshwater habitats throughout the world. These include clams, oysters, and mussels.

Mussels

Cephalopod: Cephalopods are a class of fast-swimming mollusks. Some have a hard external shell, but most do not. Cephalopods are marine predators; these carnivores eat fish, worms, crustaceans, and other mollusks. Some cephalopods include the octopus, squid, cuttlefish, nautilus, and ammonite. There are about 650 living cephalopods and over 7,500 known extinct cephalopods. Cephalopod means "head-foot." The foot of the cephalopod is a cluster of tentacles that connects directly to the head. The brain, the eyes, and the other sensory organs are well-developed. Cephalopods breathe using gills. They swim by jet-propulsion; they rapidly expel water from the mantle cavity through a specialized, tube-like organ called the hyponome. Many can also eject a cloud of ink to confuse enemies  

Octopus

LOCOMOTION IN SQUIDS

cLAW FORM AND FUNCTION