690 



ZOOLOGY 



SECT. 



vessels, many of which are devoid of proper walls and have there- 

 fore the nature of sinuses. The returning blood passes into a 

 large longitudinal vein, the vena cava {v. c.) placed between the 

 nephridia, whence it is taken to the kidneys themselves {nph. v.), 

 thence by afferent branchial veins (af. hr. v.) to the gills, and is finally 

 returned by efferent branchial veins (ef. br. v.) to the auricles. The 

 mantle has a very extensive blood supply, and, as mentioned above, 

 probably acts as the chief respiratory organ : its blood (art.^) is 

 returned directly to the auricles without passing through either 



Fig. 50'.). — Diagram of the circulatory system of Anodonta. Vessels containing aerated blood 

 red, non-aerated blue. af. br. v. afferent branchial veins ; ao. aorta ; art. 1, artery to mantle ; 

 art. 2, artery to body generally ; au. auricle ; ef.hr.v. efferent branchial veins ; nj^h.v. nephridial 

 veins ; jjc. pericardium ; v. ventricle ; v. c. vena cava. The arrows shovr the direction of the 

 current. 



the kidneys or the gills. The blood is colourless and contains 

 leucocytes. 



The nervous system is formed on a type quite different from 

 anything we have yet met with. On each side of the gullet is a 

 small cerebro-pleural ganglion (Fig. 566, c. pi. gn.) united with its 

 fellow of the opposite side by a nerve-cord, the cerebral co7ninissure, 

 passing above the gullet. Each cerebro-pleural ganglion also gives 

 off a cord, the cerebro-pedal connective, which passes downwards 

 and backwards to a pedal ganglion (pd. gn.) situated at the junction 

 of the visceral mass with the foot : the two pedal ganglia are so 

 closely united as to form a single bilobed mass. From each 

 cerebro-pleural ganglion there further proceeds a long cerehro- 

 visceral connective which passes directly backwards, through the 

 kidney, and ends in a visceral ganglion {v. gn.) placed on the ventral 



