THE CRAYFISH 259 



The excretory organs of the crayfish are known as the 



green glands. They lie in the head, immediately 

 Organs! 7 behind the antennae, upon whose basal joints 



their ducts open. The duct is connected 

 with a thin-walled bladder, below which lies a green mass. 

 This has a complicated structure, which may be summed up 

 by saying that it is a labyrinth of spaces whose walls are 

 lined by a cubical, glandular epithelium differing in char- 

 acter in different regions. In the middle of it is a small, 

 brownish sac whose cavity is divided by partitions and 

 communicates by a single opening with the labyrinth. This 

 is known as the end sac. The labyrinth has been compared 

 to a nephridium, but more probably belongs to a class of 

 mesodermal tubes known as " coelomoducts " (p. 335), of 

 which we shall meet with further examples. The end sac 

 is a minute portion of the ccelom, which is otherwise repre- 

 sented in the crayfish only by the cavity of the genital 

 organ. 



In its general plan the nervous system of the crayfish 



resembles that of the earthworm. In the front 

 system! P art °f ^ head, between the green glands, lies 



a supra-msophageal or cerebral ganglion, or brain, 

 which corresponds in position to the supra-pharyngeal 

 ganglia of the worm. It gives nerves to the eyes, anten- 

 nules, and antennas, and from it two long circumxsophageal 

 commissures pass backwards to join behind the oesophagus 

 in the subcesophageal ganglion. This gives nerves to the 

 limbs as far as the second maxillipeds inclusive, and 

 immediately behind it lies the first thoracic ganglion, which 

 supplies the third maxillipeds. In each of the remaining 

 segments of the thorax lies an indistinctly double ganglion 

 which supplies by several nerves the limbs and other 

 organs of its segment. These ganglia are set at some 

 distance apart and are connected by double commissures, 

 forming thus a ventral cord. Between the fourth and fifth 

 ganglia the commissures part widely to allow the sternal 

 artery to pass between them. In the abdomen the cord is 

 continued and consists of a ganglion in each segment united 

 to its fellows by longitudinal commissures, which are really 

 double, but appear at first sight to be single. The last 

 ganglion supplies the telson as well as its own segment. 



