Cephalization. \ i 



each other, in the adult what appear to be the verte- 

 bral segments are really due to a secondary cleaving 

 occurring in a later stage, after the originally separate 

 primary segments have become fused. 



As we ascend in the scale of complexity among 

 vertebrates, we find as a rule that the head becomes 

 more and more highly organised, and that there is a 

 tendency towards the concentration of its elements, 

 and that the fore parts of the body become more and 

 more subservient to it. This reaches its climax in man, 

 where we find the anterior pair of limbs entirely set 

 apart to wait on the head. 



There are five classes of vertebrate animals fishes, 

 amphibians, reptiles, birds, and mammals. 



CHAPTER IT V /> 



CLASS I, PISCES (fS^HEsW/OTr 



8. General characters of fishes. Fishes 

 tute the first and simplest class of the head-bearing* *. 

 vertebrates, and, like the simplest forms of all the 

 other sub-kingdoms, they are aquatic in habit, and 

 all their organs are adapted for a watery home. 

 Thus in shape they are for the most part of an 

 elongated flattened outline, pointed in front, tapering 

 behind, so as to afford as little resistance as possible 

 in traversing the water ; the fore part of the body, or 

 head, is joined to the trunk directly, without the inter- 

 vention of a narrow neck, and to the hinder ex- 



