78 COMPARATIVE ANATOMY AND PHYSIOLOGY, 



some such schematic Mollusc (Lankester) as that here 

 figured. The oblong body is bilaterally symmetrical, 

 and the prsestomium, as in Peripatus, is provided 

 with a pair of tentacles (Fig. 36, A, a) ; the mouth 

 (B, o) is on the lower surface, and near, though not 

 at the front end, while the anus (m) is median, dorsal, 



1 f 



a 



d 



B 



Fig. 36. Diagrams of the Typical Structure of a Mollusc. A, from 



above ; B, from below. 



a, Tentacles of head ; 6, head ; c, edge of mantle ; e, outline of foot seen through 

 the mantle, which is supposed to he transparent ; /, edge of shell-follicle; 

 g, shell ; h, osphradium (Sprengel's olfactory organ); i, ctenidia (gills); k, 

 generative orifice (paired) ; I, aperture of one of the nephrldia (excretory 

 organs) ; m, anus ; , foot where it extends beyond the visceral mass ; o, 

 mouth ; p, plantar surface of foot. (After Ray Lankester.) 



and posterior ; right and left of this anal opening we 

 find the orifices of the excretory organs (Z), and near 

 them those of the genital ducts (&). 



So far the creature presents no characters other 

 than such as we might expect to find in any co3lomate 

 Metazoon ; in addition, there are four characters of 

 greater significance. The ventral surface is produced 

 into a more or less triangular muscular outgrowth, 

 which is known as the foot ; the dome-like dorsal 

 surface, which contains the chief mass of the viscera, 



