108 ANIMAL FORMS 



(and known as green glands), as in the crayfishes or crabs, 

 or on the second maxillae (shell-glands) in many of the 



PIG. 65. Dissection of crayfish, b, brain ; k, heart ; i, intestine ; k, kidney ; /, liver ; 

 n, nerve-cord ; r, reproductive organ ; s, stomach, showing two teeth in position. 



simpler crustaceans. Their method of operation is much 

 like that of the kidneys in the earthworm. 



107. Nervous system and special senses. The nervous sys- 

 tem also shows a decided resemblance to that of the anne- 

 lids. The cerebral ganglia or brain is situated above the 

 alimentary canal in the head, and connects with the ven- 

 trally lying cord by a collar. As in the earthworm, this 

 ventral cord is double, and bears a pair of swellings or gan- 

 glia in each segment. In the crayfish, crabs, and other 

 highly modified forms, where the segments tend to fuse, 

 several of these ganglia may also unite, and except in early 

 life their number cannot be determined. 



Among the less specialized Crustacea the order of intel- 

 ligence is low, though perhaps it may prove to be higher 

 than is usually supposed when such forms have been more 

 thoroughly studied. The following quotation relating to 

 the lobster applies even more to the higher forms, the 

 crabs : " Sluggish as it often appears when out of water and 

 when partially exhausted, it is quite a different animal when 

 free to move at will in its natural environment on the sea- 



