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. br. 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. 1 ) is 

 returned directly to the auricles without passing through either 



FIG. 569. 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.bjtr. efferent branchial veins ; nph.r. nephridial 

 veins ; pc. pericardium ; v. ventricle ; r. c. vena cava. The arrows show 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 commissure, 

 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 cerebro- 

 visceral connective which passes directly backwards, through the 

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



