276 ELEMENTS OF COMPARATIVE ZOOLOGY. 



distinct order of Heteropoda are especially modified for a 

 life on the high seas. 



SUBCLASS II. EUTHYNEURA. 



In the Euthyneura the nervous system is without a 

 twist, and the head almost always bears two pairs of 

 tentacles. 



ORDER I. OPISTHOBRANCHIA. 



These forms are all marine, and have but two divisions to 

 the heart an auricle and a ventricle, the latter being in 

 front of the former. Some are provided with a spiral shell, 

 while others called Nudibranchs or naked molluscs are 



Fio. 127. Naked mollusc (Doris), showing the gills, above to the right. 



without such protection. In the nudibranchs there are 

 commonly developed gills upon the dorsal surface, and in 

 the living condition these forms are, from their bright colors 

 among the most attractive of molluscs. Here, too, are 

 forms (Pteropods) especially developed for a life on the sur- 

 face of the ocean, the foot being modified into a pair of 

 wing-like structures. 



ORDER II. PULMOKATA. 



The great majority of the land and fresh- water snails 

 and slugs belong here. In them gills have disappeared, and 

 the mantle-cavity has been modified into an organ (lung) 

 for breathing air, the opening to which is to be seen on the 



