GENERAL CHARACTERS OF VERTEBRATA. 51 



Articulata, its purpose appears similar, but is carried into effect in a different 

 manner, the locomotive organs being the parts chiefly supplied by it. In the 

 Vertebrata, on the other hand, the development of all the other organs appears 

 to be subordinate to that of the Nervous System ; their object being solely to 

 give to it the means of the exercise of its powers. This statement is not, of 

 course, as applicable to the lower Vertebrata, as it is to the higher ; but it is 

 intended to express the general character of the group. The predominance 

 of the nervous system is manifested, not only in the increased size of its cen- 

 tres, but also in the special provision which we here find, for the protection 

 of these from injury. In the Invertebrated classes, wherever the nervous sys- 

 tem is inclosed in any protective envelope, that envelope serves equally for 

 the protection of the whole body. This is the case, for example, in regard 

 to the spiny integument of the Star-fish, the shell of the Mollusca, and the 

 firm jointed rings of the Insect. The only exceptions occur in a few tribes, 

 in which the nervous system is much concentrated ; and in which the general 

 organization approaches that of the Vertebrata.* In Vertebrated animals, we 

 find that the skeleton essentially consists of a series of parts, which are de- 

 stined to inclose the nervous centres, and to give attachment on their exterior 

 to the muscles by which the body is moved ; hence it may be termed the 

 neuro-skeleton ; in contradistinction to the denno-skeleton, which envelopes 

 the whole body in many Invertebrata, being formed on the basis of their in- 

 tegument. The tissues, bone and cartilage, of which the former is composed, 

 are more closely connected with the vascular system, than are the hard parts 

 of Invertebrata ; and are consequently more capable of undergoing interstitial 

 change. 



21. In considering the essential character of the skeleton of Vertebrata, we 

 should look at its simplest forms, those in which it has the least number of 

 superadded parts. We find these in the Serpent tribe, among Reptiles, and 

 in the Eel and its allies among Fish. If we examine their skeletons, we per- 

 ceive that the Spinal Column, with the Cranium at its anterior extremity, 

 constitutes the essential part of the vertebrated frame-work ; and that the de- 

 velopment of members is secondary to this. The Spinal Column usually 

 consists of a number of distinct bones, the Vertebrae ; each of which is per- 

 forated by a large aperture, in such a manner that, when the whole is united, 

 a continuous tube is formed for the lodgment of the spinal cord. The Cra- 

 nium, which it bears at its upper end, is in reality formed of the same elements 

 as the vertebrae, instead of differing from them completely in structure, as we 

 might be led to suppose by examination of its most developed forms only. 

 The object of this enlargement is to inclose the brain, or mass of cephalic 

 ganglia, which attains a greatly-increased size in the Vertebrata; and also to 

 afford support and protection to the organs of special sense, which are far 

 more highly developed among them than they a're in the lower classes. The 

 true nature of the cranium is best seen in those animals, in which the brain 

 bears but a small proportion to the spinal cord, such as the lower Reptiles 

 and Fishes ; and an examination of its structure in these satisfactorily proves 

 the reality of this view, which is further borne out by the history of its de- 

 velopment, and of that of its contained parts, in the higher Vertebrata. 



22. The Vertebral column at its opposite extremity, is usually contracted 

 instead of being dilated, forming a tail, or a rudiment of one, from which 

 the nervous centres are entirely withdrawn ; the development of the tail is 



Thus, in the highest Crustacea, there is an internal projection from the shell, on each 

 side of the median line, which forms a sort of arch inclosing the ventral cord ; and in the 

 naked Cephalopoda, the nervous centres are supported, and in part protected, by cartilagi- 

 nous plates, which are evidently the rudiments of the internal skeleton of the Vertebrata. 



