MOLLUSKS 



99 



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. 55), 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- 

 turbed. A shell is by 

 no means a necessity, 

 however, for in many 



FIG. 55. The chiton, armadillo-snail or sea-cra- 

 dle. The left-hand figure shows mouth in 

 center of proboscis, the broad foot on each 

 side of which are numerous small gills. The 

 right-hand figure shows the mantle and shell, 

 composed of eight plates. From life, one- 

 half natural size. 



species, such 



as the 



beautiful naked snails 

 or Kudibranchs (Fig. 



56) 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. 



94. 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. 55) the number of gills is 

 greater, amounting in some species to over a hundred, 

 while in the Nudibranchs (Fig. 56) 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. 



