GASTEROPODA CONSIDERED AS A CLASS. 



405 



Gasteropods have a heart with one auricle and one 

 ventricle and arteries ; but veins are wanting or incom- 

 plete. 



Some kinds perform their respiration by means of gills, 

 and are called the Branchifera. Others breathe by 

 means of lung-like cavities, and are called the Pulmo- 

 nit'era. 



Their nervous system is but little more than a cephalic 

 ganglion and a thoracic ganglion united so as to form a 

 collar around the gullet. 



Their two eyes are situated on long stalks. 



Their ears are a pair of small membranous vesicles. 



The sense of smell and the sense of touch reside in the 

 tentacles. 



The Gasteropods lay eggs from which the young are 

 hatched. As already stated, however, on page 392, the 

 River-Snails (Paludina) are viviparous. The eggs of the 

 Land-Snails are protected by a flexible shell in some spe- 

 cies, and by a brittle shell in others; The eggs of the 

 Fresh-water Snails are soft and transparent. The spawn 

 of some kinds of Sea-Snails is in the form of a band or 



FIG. 550. 



NIdarnental capsules of a Gasteropod Whelk, Buccinum. 



a, single capsule showing the hole through which the five or six young eacape; 6, 

 youug shell and line indicating its natural size. 



