204 



THE SCIENCE OF LIFE. 



orous species. The former are generally land and fresh- 

 water forms, and the latter all marine. 



Gasteropods comprise three fourths of all living Mol- 

 lusks, and are representatives of the type. 



Omitting a few rare forms, as Dentalium and Carin- 

 aria, we may divide the class into the following orders : 



1. Pteropods, (Gr., pteron, wing ; pous, foot,) which are 

 small marine floating Mollusks, whose main organs re- 

 semble a pair of fins or wings, whence the common name, 

 " Sea-butterflies." Many have a delicate, transparent 

 shell. The head is said to carry six appendages, armed 

 with several hundred thousand suckers, forming a pre- 

 hensile apparatus unequaled in complication. 



2. Opisthobranchs, (Gr., opisthon, behind ; bragchia, 



FIG. 103. A. Tritonia Hombergi. B. Horned Doris. 



gills.) These are generally naked Sea-slugs, a few only 

 having a small shell. The feathery gills are behind the 

 heart, (whence the name.) They are found in all seas, 

 generally on rocky coasts. When disturbed, most of 

 them draw themselves up into a lump of jelly or tough 

 skin. These naked-gilled Mollusks (Niidibranchiatd) ex- 



