IV. GASTEROPODA. 



375 



degrees, so that the auricle is in front and the ctenidium in front 

 of this (fig. 370), while the aorta leads backwards. The nephrid- 

 ium, which communicates with the pericardium by a nephro- 

 stome, is rarely a racemose gland; usually it is saccular, the lumen 



/z. 



FIG. 368. 



FIG. 369. 



FIG. 368. I, streptoneurous nervous system of Paludina. (After Hering, from 

 Gegenbaur.) II, orthoneurous system of Linmcen. (After Lacaze-Duthiers.) 

 A, visceral; B, buccal; C, cerebral; p, pedal; PI, pleural; s6, sp, s\ib- and supra- 

 intestinal ganglia; n, olfactory nerve; p, otocyst. 



FIG. 369. Diagram of circulation in Doris. (After Leuckart.) a, auricle; c, gills 

 around anus; t, tentacle; v, ventricle; ar, vessels returning venous blood from the 

 body. 



bearing gland cells and concretions; its duct either empties into 

 the mantle cavity or beside the anus. 



The sexual organs in some forms (Cyclobranchs and many 

 Zygobranchs) empty into the nephridia. They show two extremes. 

 On the one hand are completely dioecious species, on the other 

 there may be complete hermaphroditism (many Tectibranchs, 

 Pteropoda), in which the male and female organs are united 

 throughout their extent. Intermediate stages occur; those of 

 the pulmonates are described below. 



