374 MOLLUSC A. 



usually on the right side in front, in or beside the mantle chamber. 



Itarely it empties in the middle line behind (figs. 305, 370, 371). 



(Esophagus, stomach, and intestine are slightly marked off 



from each other. The convolutions of the i'.itestine are enveloped 



Fig. 367. — Row of teeth from the radula of Troclius cinerariui^. (After Schmarda.) 



by the liver, which by its large size forms the chief part of the 

 visceral sac. A pair of salivary glands empty into the pharynx, 

 these in the Doliidaj secreting free sulj^htiric acid. 



The nervous system usually differs from that of other molluscs 

 in that the pleural and parietal ganglia are free (p. 353). If the 

 commissures be short, the ganglia are collected near the pharynx 

 and, thus freed from the body torsion, are s\-mmetrical (orthoneu- 

 rous, fig. 308, II). If the cerebrovisceral commissure be longer, 

 the result is almost always streptoneury (cliiastoneury). Pleural 

 and visceral ganglia hold their place, but the right parietal ganghon 

 crosses above the intestine to the left side (hence called supra- 

 intestinal), while the left passes under the intestine to the right 

 side (snbintestinal), the cerebrovisceral commissure being twisted 

 like the figure 8. The strong development of the pharynx is ac- 

 companied by buccal ganglia. The existence of streptoneurous 

 forms among the orthoneurons Opisthobranchs {Actceon) and Puh 

 nionata {CJiilina) shows that orthoneury in these groups has arisen 

 from streirtoneury. 



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

 genera {Ilaliotis, Fissuri'lht) recall the Acephala in having these 

 organs in pairs, while the intestine passes through the heart. As 

 a rule the asymmetry induced by the torsion of the body has 

 resulted in the loss of the ctenidium, osphradium, nephridium. 

 and auricle of one (the primitively left) side. Prosobranchs and 

 Opisthobranclis are recognized accordingly as the gills are on the 

 anterior or posterior part of the body. In the Opisthobranchs 

 (fig. 309) 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 behirid and forces it forward to the head by an aorta. 

 In the Prosobranchs the heart has been twisted about ninety 



