274 ELEMENTS OF COMPARATIVE ZOOLOGY. 



In all Gasteropods a shell is present in the young, but in 

 many it is lost before the animal becomes adult. It is 

 never a bivalve structure, but is either plate-like or is coiled 

 in a spiral. In some the spiral is flat, in others it may be 

 elongate, and the turns may be either to the right or to the 

 left, right-handed shells being in the great majority. In a 

 large number of Gasteropods a shell-like structure (oper- 

 culum) is developed on the dorsal surface of the hinder 

 part of the foot, and when the animal withdraws itself into 

 the shell this operculum closes the opening after all the soft 

 parts are inside. 



Some of the peculiarities of the nervous system form the 

 basis of the subdivision of the Gasteropods. In one group 

 (Euthyneura) the ganglia and the cords connecting them 

 are much as in the clam. In the other (Streptoneura) the 

 cords leading back from the brain become crossed so that 

 the nerve which starts from the right side goes to a ganglion 

 on the left, and vice versa. 



In all Gasteropods a lingual ribbon (p. 270) is present, and 

 this works against a plate or "jaw " on the upper side of 

 the mouth. The alimentary canal is rarely straight. 

 Usually there are convolutions, and the whole is so bent 

 upon itself that the vent is carried far forward, and may be 

 placed upon the neck just above the mouth. Sometimes it, 

 or the liver connected with it, become greatly branched. 



SUBCLASS I. STREPTONEURA. 



In these the nervous system is twisted ; there is but a 

 single pair of tentacles upon the head; and the gills are 

 placed in front of the heart, a condition which leads many 

 naturalists to call the group "Prosobranchs." 



