14 SHELL GALLERY. 



small glassy shells ; others (Gymnosomata) are naked. They exist 

 in countless millions in some parts of the ocean, discolouring the 



\ 6. 



h 



Shell-less Pteropod (Clio borealis), 



a. Dorsal view. 6. Ventral aspect. 



water for miles. They constitute the principal food of the Baleen 

 Whales. 



About a hundred species are known. 



GASTROPODA*. 



[Cases As is indicated by their name, the mollusks of this class crawl 



1e-L p. or glide on the under surface of their body, termed the "foot." 

 They have been divided into four sections : — 1. The Prosobrancbs, 

 which have the gills situated behind the head in advance of the 

 heart, and are always provided with a shell. 2. The Opisthobranchs, 

 in which the position and structure of the gills are variable, but 

 always behind the heart, aud in which the body is either naked or 

 with the gills protected by a shell, which is external or concealed in 

 the mantle. 3. The Nuclcobranchs, which have the gills in a tuft 

 at the hinder part of the back, in some cases protected by a shell ; 

 they do not crawl like the ordinary Gastropods, but are found 

 swimming freely in the open sea, like the Pteropods. 4. The Pul- 

 monata or air-breathers, the breathing-cavity of which opens 

 only by a small aperture, which can be closed by a valve. These 

 four primary divisions of Gastropods have been variously split up 

 into smaller sections — families, genera, aud subgenera ; the more 

 interesting will be pointed out in the following pages. 



* From the Greek: gaster, holly. Bud potts, foot. 



