OSMOTIC REGULATION AND EXCRETION 



211 



glandular mass and above it a thin-walled bladder from which a 

 short duct leads to the opening. In the centre of the mass is a small, 

 brownish sac, known as the end-sac. The cavity of this is a vestige 

 of the coelom, which otherwise is in the crayfish represented only 

 by the hollow of the gonad. Partitions project into it from its wall, 

 and it communicates by a small opening with the rest of the mass, 

 known as the labyrinth, which is essentially a winding and much 

 complicated tube leading from the end-sac to the bladder. Its 

 first section, which forms the 

 outer part of the gland, known 

 as the cortex, is greenish in 

 colour and broken into a mesh- 

 work of channels. The rest, the 

 medulla of the gland, is a 

 whitish, coiled tube, simple for 

 a short distance and then made 

 spongy by ridges of its wall. The 

 gland behaves as an osmotic 

 regulator in much the same way 

 as the vertebrate kidney ; a 

 filtrate from the blood is first 

 formed, containing its crystalloids 

 but not its proteins, and lower 

 down there is absorption of most 

 ions and secretion of some others. 

 The resulting fluid is of lower 

 concentration than the blood, so 

 that the body gets rid of water. 

 Some nitrogen is lost from the gland but the chief excretory organ 

 is the digestive diverticula. Certain gland cells found on the gills 

 are possibly also excretory. The principal nitrogenous excreta 

 are ammonia and amino compounds. 



trrr^^ '^^ 



e.s: 



CDK - 



Fig. 143. — A diagram of the structure 

 of the green gland of a crayfish. 

 Above, the whole gland is seen in 

 longitudinal section ; below, the 

 end sac and cortex are seen as 

 dissected out and viewed from the 

 surface. 



hi. 



Bladder ; cor., cortex ; e.s., end-sac ; med., 

 medulla ; o., opening on antenna. 



NERVOUS SYSTEM 



In its general plan the nervous system of the crayfish resembles 

 that of the earthworm. In the front part of the head, between 

 the green glands, Hes a supra-oesophageal or cerebral ganglion, 

 or brain (Fig. 144), which corresponds in position to the supra- 

 pharyngeal gangha of the worm. It gives nerves to the eyes, 

 antennules, and antennae, and from it two long circumoesophageal 



