IV. GASTEROPODA. 



■iio 



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

 of this (iis. 370), ^vhile 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 



Fig. 308. 



Fig. 389. 



Fig. 368.— I, streptoneurous nervous sy.stem of Paludimi. (After Hering, from 

 Gegenbaur.) JI, orthoneurous system of Limncen. (After Lacaze-DUthiers.) 

 A, visceral; B, buccal; i", cerebral; p, pedal; PI, pleural; sb, .s^j, sub- and supra- 

 intestinal ganglia; n, olfactory nerve; ", otocy.st. 



Fig. 3(i9.— Diagram of circulation in Dorix. (After Leuckart.) «, auricle; c, gills 

 around anus; t, tentacle; v, ventricle; x, 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 hermtiphroditism (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. 



