266 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



of the head, the successive lobes of the brain (cerebel- 

 lum, optic lobes, cerebrum, and olfactory lobes) corre- 

 spond to the successive cranial vertebrae. 



The muscular system shows segmentation in a marked 

 degree in fishes and some lizards. The flakes of a fish's 

 flesh correspond to the vertebrae, and we have here lit- 

 erally body segments (somites). See also the so-called 

 glass snake — really a limbless lizard. Under a sharp 

 blow the tail breaks into segments right through be- 

 tween scales, flesh flakes, and vertebrae. 



In the visceral system the segmentation is not dis- 

 cernible. 



As a general law, the number of segments and their 

 similarity is greater as we go down the scale of verte- 

 brates, and, contrarily, as we go up the scale adaptive 

 modification obscures more and more the homology. 



SECTION III. 

 Invertebrates. 



I. ARTICULATA. 



Under Articulata we include arthropods and annelids 

 or worms (see schedule, page 71), because, from the homo- 

 logical point of view (though not from any other), they 

 are best united. What we propose to prove is that the 

 structure of all these animals is exactly such as it would 

 be if they all came by descent, with modifications, from 

 one primal articulate animal ; and, therefore, that we 

 must conclude that they did all so come. 



General Plan. — The general plan of structure is the 

 same throughout, but wholly different from that of ver- 

 tebrates. 1. The skeleton is external, instead of internal. 

 2. The nervous system lies along the ventral, instead of 

 the dorsal, aspect of the body. 3. The body consists of 

 one cavity, instead of two. 



