xxvii GENERAL CHARACTERS 323 



On either side of the body, between the trunk and the 

 mantle, are two gills (/. G, O. G), each having the form of a 

 double plate (B) nearly as long as the body. They serve, in 

 conjunction with the mantle, as respiratory organs, but their 

 main function is to produce the current of -water referred to 

 above by means of the cilia with which they are covered. 



There is an extensive system of blood-vessels. The heart 

 lies in the ccelome, and consists of three chambers, a median 

 ventricle ( Vent), which surrounds the intestine, and paired 

 auricles (Aur). 



Excretion is performed by a single pair of nephridia 

 (Nphui) which open at one end (Np/i. st) into the ccelome 

 and at the other (Np/i. p) on to the exterior. 



The nervous system consists of three pairs of ganglia, the 

 two ganglia of each pair being united by transverse com- 

 missures. The cerebro-pleural ganglia (C. P. Gri) lie above 

 the gullet, and represent, in a general way, the brain of 

 Polygordius and the crayfish ; they are united by longitu- 

 dinal connectives with the pedal ganglia (P. Gti), which lie 

 in the foot and may be taken as representing the ventral 

 nerve-cord of worms and arthropods, and with the visceral 

 ganglia ( V. Gn) which are placed beneath the posterior 

 adductor muscle. 



The gonads (Goti) are large irregular organs, very similar 

 in appearance in the two sexes, situated among the coils of 

 the intestine and opening by a duct (Gnd] on either side of 

 the trunk, close to the nephridiopore. The impregnated 

 eggs are passed into the cavity of the outer gill of the 

 female, where they undergo the early stages of their develop- 

 ment. The larva of the fresh-water mussel is a peculiar 

 bivalved form, very unlike the adult, and called Qglochidium ; 

 but in the more typical molluscs the embryo leaves the egg 

 as a trochosphere, closely resembling that of Polygordius. 



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