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, 



* I 



d 



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

 above ; B, from below. 



a, Tentacles of head ; 5, 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, osphradiiiiu (Sprcngel's olfactory organ); i, ctenidia (gills); fc. 

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

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

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



and posterior ; right and left of this anal opening we 

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

 them those of the genital ducts (k). 



So far the creature presents no characters other 

 than such as we might expect to find in any ccelomate 

 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, 



