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 Scaphopoda 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- 
cypoda 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, ete. 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, ete. 
CLASS AMPHINEURA 
ORDER POLYPLACOPHORA 
The Amphineura, 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 
branchiz 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. 
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