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CLASS V. PTEROPODA. 



CONSIDERED numerically, this Class is perhaps 

 the least important in the animal kingdom, as it 

 comprises but a very few species, either recent or 

 fossil, and those of small size. Of these the British 

 seas have hitherto yielded but five, neither of which 

 can be considered other than as an accidental strag- 

 gler to our shores. They are all, in fact, oceanic ; 

 unlike the BRACHIOPODA, which are always fixed, 

 these are invariably found swimming at large in 

 the open sea, being unprovided with any means 

 of attaching themselves even for a moment, or of 

 crawling on a solid support. 



The most prominent character in this Class is 

 the possession of two broad muscular fins, one on 

 each side of the neck, somewhat resembling the 

 expanded wings of a butterfly, whence Cuvier gave 

 them the name of PTEEOPODA, or " wing-footed." 



An advance in structure above the molluscous 

 animals already considered is shown in the pre- 

 sence of a distinct head, with tentacles, and jaws, 

 and various organs of sense. In Clio, whose ana- 

 tomy has been most investigated, there is an extra- 

 ordinary apparatus developed for seizing its minute 

 prey. On each side of the mouth are three fleshy 

 warts, covered with minute red specks. Under a 

 powerful microscope these specks, which number 

 about three thousand on each tentacle, are seen to 

 be transparent cylinders, each containing in its 

 cavity twenty stalked disks, which may be pro- 



