CLASS LAMELLIBRAXCHIATA. 



57 



" valves " (Latin, valvce, folding-doors), and wliicli is for 



this reason said to be " bivalve." Tliey derive their 



scientific name, on the other ^ 



hand, from the fact that 



they breathe by means of 



gills (Greek, hrangckia, a 



gill), and that these gills 



have a flattened, plate-like, 



or "lamellar" form (Latin, 



lamella, a flat plate). 

 As the type of this class 



we shall take the great 



Sand Gaper {Mya arena- 



ria) ; not because it is espe- 

 cially common, but because 



it exhibits particularly well 



many of the characters of 



the Bivalves. The body 



of the Gaper is quite soft, 



and may be regarded as a 



kind of sac or bag, formed 



by the skin. This bag is 



termed the "mantle," be- 

 cause it conceals and pro- 

 tects the internal organs 

 within it like a cloak. 

 The bag formed by the 

 mantle has two openings 

 into it. One of these open- 

 ings is placed at the hinder 

 end of the body, and serves 

 as an aperture by which water is admitted into the interior 

 of the body. At this opening the sac is drawn out into 

 a long tube (fig. 24, s), which is really double, and which 

 the animal can thrust out and draw in again. The other 

 opening into the mantle-sac is for the purpose of allowing 

 what is called the "foot" to be thrust out — this really 

 being a muscular, tongue-shaped organ, by means of which 

 the animal can shift its position (fig. 24,/). 



Rg. 23. — Diagrammatic vertical and 

 transverse section of Mya arenaria. 

 b Back, or "dorsal margin" of the 

 shell ; s s The two valves of the shell, 

 right and left ; m m The two halves, 

 or " lobes," of the mantle, producing 

 the shell ; jr g The gUls, two pairs on 

 each side ; h The heart ; i Intestine ; 

 / The foot. 



