GASTROPODA 493 



ings of the digestive, excretory, and genital organs. In some snails, 

 especially among the nudibranchs, the mantle is wanting; the shell is also 

 absent in these animals, and the gills, when present, are not homologous 

 to the ctenidia of other mollusks, but are simple outgrowths of the 

 integument of the back. 



Internal Structure. The spiral twisting of the visceral mass of most 

 snails has caused a displacement of the internal organs and the loss of 

 the left-hand member of the primitively paired ones. Thus, but a single 

 ctenidium, kidney, auricle, and osphradium are usually present, instead of 

 a pair. The mouth opens into the pharynx (Fig. 742), which contains 

 the tongue and the radula. In the roof of the pharynx and opposed to 

 the tongue are one or more cuticular plates called the jaws, against which 

 the radula is rubbed in the process of rasping. A pair of salivary 

 glands pour their secretion into the pharynx. The alimentary tract, 

 which consists of the oesophagus, the stomach, and the intestine, bends 

 on itself, the anus being not far from the mouth of the shell. 



Three classes of respiratory organs are found ctenidia, lungs, and 

 adaptive gills. In most cases ctenidia (Fig. 821) are present; pulmo- 

 nates, both aquatic and terrestrial, breathe by means of lungs; nudi- 

 branchs have neither ctenidia nor lungs, but in their place adaptive gills 

 (Fig. 765), which are projections of the dorsal body wall. A few nudi- 

 branehs do not possess these organs, but respire through the ciliated 

 integument. The heart bears a close relation to the gills. Certain primi- 

 tive snails, which possess a pair of ctenidia, have also two auricles, while 

 the majority of them, having but a single ctenidium, have also but one 

 auricle. Nudibranchs and pulmonates have also but one auricle. In the 

 opisthobranchs and the pulmonates the gills or lungs are behind, while 

 in the prosobranchs, as the name indicates, they are in front of the heart. 



The excretory organs consist of a single kidney, which opens into 

 the mantle cavity near the anus; it also has a communication with the 

 pericardial (ccelomic) cavity. Those snails which have two ctenidia have 

 also a pair of kidneys. 



The nervous system contains the characteristic ganglia. The cere- 

 bral ganglia are above and the pedal ganglia below the oesophagus and 

 united by commissures. The pleural ganglia are usually between and 

 joined with these two. The visceral ganglia lie below the intestine and 

 are joined with the pleural, and the parietal are near the visceral. In 

 the prosobranchs and in Acteon, 'an opisthobranch, and Chilina, a pulmo- 

 nate, the commissures joining the visceral with the pleural ganglia cross 

 each other in such a way that the nerve from the right pleural passes 

 over the intestine to the left side of the visceral. Those gastropods in 

 which this occurs are called streptoneurous ; the others are called euthy- 



