108 ANIMAL FORMS 
(and known as green glands), as in the crayfishes or crabs, 
or on the second maxille (shell-glands) in many of the 
Fic. 65.—Dissection of crayfish. 0, brain; A, heart; é, intestine ; %, kidney ; 2, liver; 
m, nerve-cord ; 7, reproductive organ ; 3, 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- 
