No. 522] 



THE CANADIAN OYSTER 



355 



view that bivalves have been developed from primitive, symmetrical, 

 gastropod-like ancestors, with a simple head in front bearing two 

 tentacles, two eyes, and mouth with a rasping tongue: a low conical shell 

 above that could be drawn down over all the soft parts and lined by 



below; and two feather-shaped gills, disposed right and left, projecting 

 backwards. Each of these was a symmetrically constructed, bipeetinate 



upper and a lower. There are -till living limpet-like gastropods along 

 our coasts possessing such characters, although no one species retains 

 them all or in the most primitive form. 



The group of mollusca to which the oyster belongs has in the course 

 of time suffered modifications of the characters mentioned. Its mem- 

 bers have taken to a more quiescent mode of life, such as burrowing in 

 sand or fixing to rocks, and in consequence have largely lost those ex- 

 ternal organs of locomotion, plunder and special sense so necessary to 

 free-roving animals. The absence of a head has been regarded as char- 

 single piece they have developed a shell with two valves (Bivalva) ; the 

 foot in by far the greatest number of forms has become somewhat 

 hatchet-shaped (Pelecypoda) ; the gills are leaf -like, each separable 

 into flat plates (Lamellibranchia). I f all bivalves had become equally 

 modified it would certainly have been difficult to determine the origin 



the reactions of these upon living organisms, certain species have been 

 forced to pursue special lines of action in order to better their chances 



studied the Lamellibranch gill. Lankester. Hatschek, Thiele, Lang, 

 the phylogeny. 



