114 



THE FRESHWATER MUSSEL. 



5. The circulatory system. 



a. The pericardial cavity is very much larger and has 

 thinner walls than in the previous section : it is 

 triangular in shape. 



PlO. 30. Anodonta cygnea. Transverse section of the animal passing 

 through the middle of the length of the ventricle. The cavity of 

 the heart is dotted, (a. m. m.) 



A, shell. AU, left auricle. B, ligament. BC, branchial cavity. F, foot. 

 I, intestine. |G, left inner gill. , left kidney. KO, Keber's organ of the 

 right side. LM, left mantle-lobe. OG, left outer gill. PC, pericardial cavity. 

 R, cavity of rectum. RG, genital gland, ovary, or testis. RM, edge of right 

 mantle-lobe. SC, supra-branchial canal of right outer gill. T, typhlosole. U, 

 left ureter, y, wall of ventricle. V', cavity of ventricle. VC, vena cava. W, 

 supra-branchial canal of left inner gill, x, free upper edge of the inner lamella 

 of the right inner gill ; between this edge and the body is the slit through which 

 the branchial cavity communicates with the supra-branchial canal of the inner 

 gill. Y, the right oerebro-visoeral connective. 



