While the origin of this unusual developmental scheme is unclear, the result is an animal which can more efficiently fit into a smaller conch opening than its close relatives, the monoplacophorans. Snail heads are frequently well developed, with effective radular structures, and extensive sensory organs. Some types of snails are free swimming, while others are sessile. Snails are found in oxygenated marine environments, freshwater, and terrestrial environments. Predatory, herbivorous, suspension feeding, deposit feeding, and parasitic species of gastropods are all known.
Cephalopods are active necto-benthic carnivores.
The body has been rotated through 180 degrees so that the anus is below the head, and the foot and head have been merged to form a cephalopedal mass. A series of tentacles ring a mouth, which frequently has a strong beak. The earliest cephalopods had an external shell attached to the dorsal side of the animal. As the organism grew, it moved its soft parts down the shell, and added septa behind it, thus defining a series of closed off chambers. Communication with these vacated, air-filled chambers was maintained by a tube called a siphuncle which ran through each of the septa.