62 INTKODUCTION TO CLASSIFICATION. 



complete, in fact, the substance of the centre of its floor, beneath 

 the primitive median line of the embryo, becomes differentiated 

 into a rod-like cellular structure, which tapers to both its ex- 

 tremities; and, in a histological sense, remains comparatively 

 stationary, while the adjacent embryonic tissues are undergoing 

 the most rapid and varied metamorphoses. 



To these great differences between vertebrates and in- 

 vertebrates, in their early condition, many others might be 

 added. In all Veriebrata that part of the wall of the body 

 which lies at the sides of, and immediately behind the mouth, 

 exhibits a series of thickenings parallel with one another and 

 transverse to the axis of the body, which may be five, or more, 

 in number, and are termed the " visceral arches." The inter- 

 spaces between these arches becoming thinner and thinner, are 

 at length perforated by corresponding clefts, which place the 

 cavity of the pharynx in free communication with the exterior. 

 Nothing corresponding with these arches and clefts is known in 

 the Invertebrata. 



A vertebrated animal may be devoid of articulated limbs, 

 and it never possesses more than two pair. These limbs always 

 have an internal skeleton, to which the muscles moving the 

 limbs are attached. Whenever an invertebrated animal possesses 

 articulated limbs, the skeleton to which the muscles are at- 

 tached is external, or is connected with an external body 

 skeleton. 



When an invertebrated animal possesses organs of mastica- 

 tion, these are either hard productions of the alimentary mucous 

 membrane, or are modified limbs. In the latter case there 

 may be many pairs of them numerous Crustacea, for example, 

 have eight pairs of limbs devoted to this function. In no verte- 

 brated animal, on the other hand, are limbs so modified and 

 functionally applied, the jaws being always parts of the cephalic 

 parietes specially metamorphosed, and totally distinct in their 

 nature from the limbs. All vertebrated animals, finally, possess 

 a distinct vascular system, containing blood with suspended 

 corpuscles of one kind, or of two, or even three, distinct kinds. 

 In all, save one, there is a single valvular heart the vessels of 

 the exception, Amphioxus, possessing numerous contractile dila- 



