400 



MANUAL OF ZOOLOGY. 



membranous processes or " palpi " (usually four in number), 

 but there is no dental apparatus. The mouth opens into a 



gullet, which conducts to a dis- 

 tinct stomach. On the right 

 side of the stomach, and open- 

 ing into it, is, in many cases, a 

 blind sac containing a peculiar 

 transparent glassy body, which 

 is known as the " crystalline sty- 

 let," but the functions of which 

 are absolutely unknown. The 

 intestine has its first flexure 

 neural, generally perforates the 

 wall of the heart, and terminates 

 posteriorly in a distinct anus, 

 which is always placed near the 

 respiratory aperture. The liver 

 is large and well developed, but 

 there are no salivary glands. 



There is always a distinct 

 heart, composed either of an 

 auricle and ventricle, or of two 

 auricles and a ventricle. The 

 ventricle propels the blood in- 



Fig. 209. - Diagrammatic vertical and tO the artCHCS, by which it is 



transverse section of Mya arenaria. distributed through the body. 

 b Back, or dorsal margin of the _, , . . J 



shell; **The two valves of the shell,. From the arteries it passes into 



arrl ic 

 and IS 



right and left; in in The two halves, 

 or "lobes," of the mantle, producing 



the shell ; gg The gills, two pairs on 



/Th e to e t. ;ATheheart; ' Inlestine; 



the gills, where it is aerated, 

 and is finally returned to the 

 auricles. 



The respiratory organs in all the Lamellibranchiata consist 

 typically of two lamelliform gills, placed on each side of the 

 body (fig. 208, b and fig. 209, g). In some cases there is only 

 one gill on each side of the body, the external pair of branchiae 

 being absent. The gills are in the form of membranous plates, 

 composed usually of tubular rods, which support a network of 

 capillary vessels, and are covered with vibrating cilia, whereby a 

 circulation of the water is maintained over their surfaces. In 

 some bivalves the margins of the mantle are united to one an- 

 other, so that a closed branchial chamber is produced ; and in 

 the others the arrangements for the admission of fresh and the 

 expulsion of effete water are equally perfect, though there is no 

 such chamber. In those in which the mantle-lobes are united at 

 their margins, there are two orifices, one of which serves to admit 



