IV. GASTEROPODA 



33] 



supraintestinal), while the left passes under the intestine to the right side 

 (subintestinal), the cerebrovisceral commissures being twisted like the 

 figure 8. The strong development of the pharynx is accompanied by 

 buccal ganglia. 



Gills, heart, and nephridia are best treated together. Certain genera 

 (H allot is, Fissurella, etc.) recall the Acephala in having the heart traversed 

 by the intestine, the paired ctenidia, nephridia and nephridial ducts, and 

 two auricles to the heart. As a rule the asymmetry induced by the torsion 

 of the body has resulted in the loss of the ctenidium, osphradium, and 

 nephridium of the primitively left side, and with the loss of one gill there 



FIG. 338. Anatomy of Cyprcca tigris (after Quoy et Gaimard). br, ctenidium ; 

 c, heart; df, vas deferens, h, liver; in, stomach; N, cerebral ganglion; oc, eye; pe, penis; 

 ph, pharynx, the radula drawn out; r, rectum; re, nephridium; t, testes. 



is usually a loss of the corresponding auricle. Prosobranchs and Opistho- 

 branchs are recognized accordingly as the gills are on the anterior or 

 posterior part of the body. In the Opisthobranchs (fig. 337) the ctenidia 

 have been lost and are replaced by secondary gills on the back. Here the 

 heart is in front of the gills; it receives blood from behind and forces it 

 forward to the head by an aorta. In the Prosobranchs the heart has been 

 twisted so that the auricle and the ctenidium are in front (fig. 338). 

 while the aorta leads backwards. The nephridium, which communicates 

 with the pericardium by a nephrostome, is usually saccular, its duct 

 empties beside the anus. 



