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 theoretical!}^ 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, fleshy ribbon that bears upon its 

 upper surface numerous rows of sharp teeth ; these rows of teeth 

 run cross ways 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 



