OYSTERS, ETC. 53 



two parts, either double or single, and the orifices are 

 surrounded by tentacles. 



Circulation. The blood is purified (aerated) and food 

 obtained by the same action. Water is drawn into the 

 siphon opening farthest from the valve (Fig. 56, in\ and, 

 wafted along by the cilia, is thus brought in contact with 

 the gills. The food-particles in the water are carried along 

 to the toothless mouth (Fig. 55, o), that, guarded by two 

 pairs of sense organs (palpi), is placed at the end of the 

 shell opposite the siphon. They then pass into the stom- 

 ach and intestine that winds about and passing through 



FIG. 56. Bivalve with siphon, showing the foot. /, ligament ; F, foot ; m, 

 mantle ; s, siphon ; ex., excurrent orifice ; /., incurrent orifice. Except 

 the siphon and the large foot, the arrangement of organs is similar to 

 that of the oyster. (After Morse.) 



the heart finally connects with the siphon nearest the valve 

 (Fig. 56, ex.), out of which all rejectementa passes. Extend- 

 ing through the body is a glass-like rod, that is possibly a 

 brace or support. The heart lies near the hinge (Fig. 55, 

 h\ and is composed of one ventricle and one auricle in 

 the oyster. In other species the heart is three-chambered, 

 or there may. be two hearts of two chambers each. Arte- 

 ries lead from the ventricle over the body, and veins carry 

 blood to the gills g y where it is purified by the air in the 

 water then passing back to the auricle. The nervous 

 system consists of three pairs of ganglia cerebral, pedal, 

 and a pair that send nerves to the internal organs, gills, 



