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The Gastropoda, the largest and most varied group of mollusks, includes 
snails, slugs, sea-hares, and limpets. They are found in marine and 
fresh waters and onland. They have retained the primitive flat ventral 
foot adapted for crawling, but inother ways have evolved significantly from 
the ancestral type. They have all undergone a torsion in the general body 
plan so that the digestive tract is no longer a straight tube, but the anus 
comes to lie in the side of the animal, often near the head. Most gastropods 
have a coiled shell and correspondingly coiled visceral mass. In some 
groups, that is, many opisthobranchs and the land slugs, the shell has be- 
come so reduced as not to appear externally at all. In some cases it is a 
small bit of calcareous material enclosed in the mantle; in other cases it 
has been lost entirely. This reduction of the shell has been the result of a 
long continued evolution; slugs are not snails that have crawled out of their 
shells. The shells of the limpets have lost the spiral structure and present 
a low conical shape. 
The Scaphopoda are burrowing mollusks having a conical foot which, by 
alternating extensions and contractions, pulls the animal through the sub- 
stratum. The mantle and shell are tubular and open at both ends. The 
shell is long and tapering and accounts for the common name ofthis group, 
the tusk shells. They are marine mollusks which do not carry on respira- 
tion by means of gills but by folds in the mantle lining. 
The Pelecypoda, or Lamellibranchia, have an axe-shaped foot adapted for 
crawling or burrowing, and have completely lost the head and the buccal 
apparatus used by other mollusks in obtaining food. They are either 
marine, brackish, or fresh-water, and feed on micro-organisms that are 
swept in contact with them by water currents created by fine hairs on the 
gills. The body is enclosed by two symmetrical mantle flaps which secrete 
right and left shell valves that are held together by a tough ligament. 
Because of this arrangement of the shell they are often referred to as 
“bivalves.'' This group includes the clams, oysters, and mussels, as well 
as the smallest pill-clams. 
The Cephalopoda include the squids, octopuses, and nautiluses and are the 
most highly evolved of the mollusks. The foot has become divided into a 
number of prehensile "arms" or tentacles arranged symmetrically around 
the head or mouth, and from this close union of head and foot the class gets 
its name. A part of the foot is further modified to form a funnel which is 
used in swimming. By forcing water out of the mantle cavity through this 
funnel, the animal achieves water jet-propulsion, All cephalopods are 
marine and in many, such as the squid and octopus, the shell is internal 
or even lost, 
