THE ARTHROPODA 321 



in constant motion, send water forward to the gill chambers. The blood 

 thus comes in contact with fresh water, is aerated, and gives off its car- 

 bon dioxide. Some of the gills are on the appendages themselves, these 

 being the podobranches ( ), while those on the basal 



part of the appendix are called arthrobranches ( ), 



on account of being on the joint itself, while those which originate on 

 the body-wall are the pleurobranch.es ( ). 



THE EXCRETORY SYSTEM 



Contrasting interestingly with many of the other animals studied 

 in the laboratory, the excretory organs of the crayfish are in the head 

 region. They consist of two rather large green glands (Fig. 207), just 

 in front of the oesophagus, with a thin-walled dilated portion called the 

 bladder, and a duct opening to the exterior through a pore at the top of 

 a little elevation on the basal segment of the antenna. 



THE NERVOUS SYSTEM 



The nervous system (Fig. 208, B) is very much like that of the 

 earthworm. The central nervous system is made up of a ventral chain 

 of nerve ganglia, though it lies dorsal to the central blood vessel. The 

 ventral chain possesses a ganglion for practically every segment, from 

 its posterior end forward. The seventh is called the sub-oesophageal 

 ganglion. 



The brain sends nerves to the eyes, antennules, and antennae. The 

 sub-oesophageal ganglion, lying in segment seven, is made up of the 

 ganglia from segments three to seven fused together. These send nerves 

 to the mandibles, maxillae, and first and second maxillipeds. Visceral 

 nerves are also supplied from the brain, extending posteriorly to the 

 viscera. 



THE SPECIAL SENSE ORGANS 



Each eye .(Fig. 208, A) is made up of some 2,500 little square facets. 

 The long rod extending immediately behind each facet is called an om- 

 matidium. It is supposed that the crayfish can thus see moving objects 

 much better than it could did it have an eye similar to higher forms. 

 But there being so many facets, it is assumed that the animal obtains 

 what is called a mosaic image, an image made up of a great many sepa- 

 rate and distinct views. However, as Latter says, "We must not con- 

 fuse this image that we think the animal obtains with the impression 

 that is given it, for the human eye sees an inverted image but the im- 

 pression is just the opposite." 



Although each ommatidium has a small range of vision and forms 

 a stiple or mosaic image, it has been calculated that the range of adjoin- 

 ing ommatidia overlaps so that a continuous picture or image is formed. 



