MOLLUSCS. 



199 



In all gasteropods a lingual ribbon (p. 195) is present, 

 and this works against a plate or 'jaw' on the upper side 

 of the mouth. The alimentary canal is rarelv 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. Some- 

 times it, or the liver connected with it, becomes 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.' 



ORDER I. DIOTOCARDIA. 



In these forms the body retains 

 its bilateral symmetry to a consider- 

 able degree, and externally may ap- 

 pear perfectly symmetrical. The name 

 implies the existence of two auricles to 

 the heart. In the limpets (fig. 37) 

 the shell is a flattened cone; in the 

 abalones it is somewhat ear-shaped 

 and very weakly spiral, but in the top 

 shells it is strongly spiral. The 

 abalones alone have any economic 

 value. Their shells, remarkable for FIG 37> _~ ei (Acmoea 

 having a series of holes in them, 

 are composed of a greenish mother- 

 of-pearl, which is extensively use 1 in inlaid work. 



From Bin- 



