MOLLUSKS 317 
tionary history of the development of the various molluscan orders 
through geological time does not seem to bear out the fact that an | 
“ideal” mollusk, as we shall construct him, ever actually existed, 
or that his prototype, the Amphineura, was the first and conse- 
quently the ancestral type of the phylum. We must therefore 
only accept this ideal mollusk theoretically, to serve as a guide 
for the time being. We may profitably use it only as a lay 
figure—a sort of manikin, which will introduce us to the phylum. 
After our introduction we must forget our imaginary type and 
abandon any notion we may have conceived of the parental posi- 
tion of the Amphineura, its closest imitator in actual life. 
THE IDEAL MOLLUSK 
The ideal mollusk has a compact, somewhat elongated body, 
the under or ventral portion of which is thickened into a mus- 
cular disk, upon which it creeps along. This disk is called the 
foot. (Note a snail thus crawling upon his “foot.”) The foot is 
rounded or obtusely pointed posteriorly. Forward, the body is 
slightly narrowed into a neck, and then expanded again into a 
head. Upon this head are carried two projecting, mobile, fleshy 
tentacles, one upon each side, that superficially resemble two 
horns. At the base of each tentacle, upon the inner side, is an 
eye. Just below and between the tentacles is a small horizontal 
slit, the mouth,—the lips being slightly thickened. The visceral 
portion of the animal is above the foot, or dorsal to it, and ex- 
tends the whole length of the animal from the mouth to the 
excretory opening, the latter being situated posteriorly. The 
visceral cavity is inclosed above by a thin skin; indeed, it is but 
the great thickening of this skin below that constitutes the foot. 
The digestive tract, beginning at the mouth, opens immediately 
into a muscular throat, which is called the buccal mass. Within 
‘the buccal mass there is a long, fieshy ribbon that bears upon its 
upper surface.numerous rows of sharp teeth; these rows of teeth 
run crossways on the ribbon, each row containing more or less 
prominent median teeth, with several laterals upon each side, and 
with marginal teeth on each side of the laterals. This is called 
the radula, and is used for tearing or rasping the food. When 
