XII 



PHYLUM MOLLUSCA 



673 



ot 



of which are devoid of proper walls and have therefore 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.l) is returned 

 directly to the auricles without passing through either 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. 576, 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 side 

 of the posterior adductor muscle. The visceral, like the pedal 

 ganglia, are fused together. The cerebro-pleural ganglia supply 

 the labial palps and the anterior part of the mantle ; the pedal 

 the foot and its muscles ; the visceral the enteric canal, heart, gills, 

 and posterior portion of the mantle. 



It will be seen that the cerebral commissures and cerebro-pedal 

 connectives, together with the cerebro-pleural and pedal ganglia, 

 form a nerve-ring which surrounds the gullet : the cerebro-pleural 

 ganglia may be looked upon as a supra-oesophageal nerve mass 

 corresponding in part with the brain of Annelids and Arthropods, 

 and the pedal ganglia as an infra-cesophageal mass representing the 

 ventral nerve-cord. 



Sensory organs' are poorly developed, as might be expected in 

 an animal of such sedentary habits. In connection with each 

 visceral ganglion is a patch of sensory epithelium forming the 

 so-called olfactory organ or, better, ospkradium, the function of 





FIG. 580. Statocyst of Anodonta. a, b,c, c', 

 ce n u lar layers surrounding the statocyst ; 

 ot - statolith. (From the Cambridge Natural 

 History.) 



