MOLLUSKS 321 



ferred to as the foot), as the common garden-snail, the peri- 

 winkles, and in general all those mollusks which have a spirally 

 coiled shell. The 8capkopoda have a long, worm-like foot, with 

 which they burrow in the sand or mud. Their shells are like 

 miniature elephant-tusks, but are open at both ends. The Pele- 

 cijpoda have a more or less club-shaped foot, utilized, in the many 

 families, for a great variety of purposes. They are always in- 

 closed in a bivalve shell. Familiar examples are the oysters, 

 the clams, mussels, etc. The Cephalopoda have the foot modified 

 into a number of arms, which encircle the head or the mouth. 

 They are the cuttlefishes, the octopi, squids, etc. 



CLASS AMPHINEURA 



ORDER POLYPLACOPHORA 



The Amphinewa, as already observed, approach most closely 

 to the ideal mollusk just described. They are bilaterally sym- 

 metrical. This fact is so important that it constitutes them a 

 class, notwithstanding the fact that in respect to the foot (the 

 basis of division into classes) they would be included with the 

 Gasteropoda, for (barring some exceptional instances) they creep 

 along upon a foot quite as our ideal mollusk would, and as the 

 Gasteropoda do. The head carries no tentacles, thus essentially 

 differing in this respect from the ideal form. The mantle is 

 extended down in front, completely covering the head. The 

 branchia3 are confined to a few pairs of ctenidia, or plume-like 

 gills, within the mantle groove or cavity, and are arranged upon 

 each side of the excretory opening like small feathers. 



There are two orders of the Amphineura, the Polyplacophora 

 and the Aplacophora (or Solenogastres), the one name mean- 

 ing "bearing many plates," and the other "without plates," 

 the word " plate " in this sense being synonymous with " shell." 

 The shell of the first order consists of eight calcareous disks 

 arranged in a longitudinal row along the back or dorsal side of 

 the animal, which overlap like shingles on a roof and admit of 



great variation of form in the various families. 

 21 



