I50 Elementary Zoology. 



clearly marked off from the connectives ; in the second place, 

 there is less correspondence between ganglia and segments 

 than in the earthworm. It will be remembered, however, that 

 the segmentation of the crayfish has not been so distinctly 

 preserved as in the earthworm. The excretory organs have 

 nearly, or perhaps quite, lost their segmented character; the 

 same is the case with the thoracic and cephalic segments, etc. 

 Hence it is not surprising to find that the nervous system 

 has shared in this confusion of an originally simple meta- 

 merism. But it is worthy of note that the nervous system, 

 inherited from much more remote ancestors than those which 

 acquired the special characteristics of the Arthropoda, has 

 retained so completely, on the whole, its regular metamerism. 



The anodon at first sight differs from the types considered. 

 The equal development of three triangularly situated pairs of 

 ganglia tend to confuse the likeness to the worm and to the 

 crustacean. The anodon, it must be borne in mind, is not a 

 segmented animal; there is no trace of segmentation any- 

 where. Hence it could not be expected that the nervous 

 system should be segmented, and it is not. But we have the 

 supra-oesophageal ganglia connected by a circum-cesophageal 

 commissure with a ventral pair, exactly as in Lumbriais and 

 Astaais. There is, however, in accordance with the absence of 

 segmentation, only one ventral pair. As for the parietovisceral, 

 they are probably to be compared to the visceral nervous system 

 of the two animals mentioned. The same observations apply 

 to the snail. 



Alimentary Canal. 



The alimentary canal of Hydra is a blind tube, formed of 

 the endoderm, and in close contact with the ectoderm, separated 

 from it only by the structureless supporting lamella. In all 

 the higher animals (with a few exceptions) there is an alimen- 

 tary canal present, which is a tube running from end to end of 

 the body, and usually opening at the posterior end by an anus 

 as well as anteriorly by the mouth. The simplest expression, 



