NERVOUS SYSTEM. 99 



The pericardial cavity is developmentally a part 

 of the body-cavity or coelom. Hence in the mussel, 

 as in the earthworm and in many other animals, 

 the kidneys are tubular organs with glandular walls, 

 leading from the body-cavity to the exterior. 



From the external apertures the excretory pro- 

 ducts are carried back by the outward respiratory 

 stream along the supra-branchial passages to the 

 cloacal cavity, and so out. 



E. The Nervous System. 



The nervous system of the mussel consists of three main 

 pairs of ganglia widely distant from one another, but united 

 by nerve-connectives, and giving off nerves which supply the 

 various organs. 



The ganglia are about the size of large pin-heads, and 

 are of a bright orange colour, which renders them easy to see. 



1. The cerebral ganglia are placed one on each side of 



the hinder border of the mouth, just above the line 

 of attachment of the mantle-lobe, and below and in 

 front of the protractor muscle. 



The ganglion lies very close to the surface, just beneath 

 the skin, and is readily exposed if its exact position 

 be determined first. 



The two cerebral ganglia are connected together 

 by a commissure which runs round the front of the 

 mouth. Each ganglion also gives nerves to the palps, 

 the anterior adductor muscle, and other adjacent 

 parts, and is joined to the other ganglia of its side by 

 nerve-connectives. 



The cerebral ganglia of the mussel probably 

 correspond to the cerebral (supra-oesophageal) and 

 pleural ganglia of other molluscs, such as the snail. 



2. The pedal ganglia are a pair of closely apposed 



ganglia about as large as the cerebral. They are 

 situated in the foot, just above the junction of the 

 visceral portion with the foot proper, and about 



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