82 ANIMAL FORMS 
many modifications. A foot is generally present, also a 
more or less well-developed head, and the body is usually 
surrounded by a shell which varies widely in shape and 
size in different species. In the common limpets the early 
coiled shell is transformed into an uncoiled cap-like one, 
and in the keyhole limpets is perforated at its summit. The 
chitons or armadillo- 
snails (Fig. 49), often 
found associated with 
the limpets, carry a 
most peculiar shell con- 
sisting of eight plates, 
which enables the ani- 
mal to roll up like an 
armadillo when  dis- 
Fie. 49.—The chiton, armadillo-snail or sea-cra- tyrbed. A shell is by 
dle. The left-hand figure shows mouth in i¢ 
center of proboscis, the broad foot on each no means a necessity, 
side of which are numerous small gills. The however, for in many 
right-hand figure shows the mantle and shell, : h th 
composed of eight plates. From life, one- Species, suc as e€ 
half natural size. beautiful naked snails 
or Nudibranchs (Fig. 
50) common along our coasts, it may be entirely absent, 
or, as in the ordinary slugs, reduced to a small scale em- 
bedded in the skin. 
83. Respiration.—A considerable quantity of oxygen is 
absorbed through the skin, as in all mollusks, but the chief 
part of the process is usually taken by the plume-like gills, 
one or two in number, which are located in the mantle 
cavity. In the chitons (Fig. 49) the number of gills is 
greater, amounting in some species to over a hundred, 
while in the Nudibranchs (Fig. 50) gills are absent, their 
places being taken by more or less feathery expansions of 
the skin on the dorsal surface. 
Many of the gasteropods left exposed on the rocks by a 
retreating tide retain water in the mantle cavity, from 
which they extract the oxygen until submerged again. 
