198 SYSTEMATIC ZOOLOGY. 



head, which usually bears sensory tentacles, and the eyes 

 are commonly placed at the bases or on the tips of one 

 pair of these structures. In some cases, as in most land- 

 snails, these tentacles can be pulled back into the body 

 in much the same way that one inverts the finger of a 

 glove. 



In the majority of forms gills or branchice are developed 

 in the mantle-chamber. In a few there is a pair of these 

 organs, but in many one gill disappears, while in other 

 species both true gills entirely disappear, and are either 

 replaced by secondary gills developed on the back or in 

 other regions; or the mantle-chamber may be richly lined 

 with blood-vessels and thus be converted into an organ 

 (lung) for breathing air. This is the case in all of our 

 common land-snails. 



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 conical, 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 (operculum) is developed on the dorsal surface of 

 the hinder part of the foot, and when the animal with- 

 draws itself into the shell this operculum closes the open- 

 ing like a door 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 our diagram (fig. 34). In the other (Strep- 

 toneura) 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. 



