THE ANNELIDES OE ENTOMOZOARIA. 401 



plete, as to embrace the adult formulas of all existing species of 

 the family, so that to convert the dentition of one species into 

 another, nothing is ever added, but merely a something left out. 

 Contrary, then, to the theories of those who maintain that the 

 adult alone is perfect, we find that it is the young which, typify- 

 ing the genus, is entitled to that character. Lastly, it is in them 

 that a portion of the nervous system acquires a development 

 equal to the production of electricity ; and in certain of the class 

 it is that we find the remains of a skeleton, the tegumentary, 

 which in the extinct world prevailed to an extent of which we 

 now can scarcely form an idea. 



" In the ganoid or mailed extinct fishes, the skeleton which, 

 from its connexion with the skin, may be called tegumentary or 

 dermoid (but to which probably the more appropriate name of 

 neuro-skeleton, as protecting the extremities of the nervous 

 papillae, might be better applied), reached its highest development 

 as regards the class fishes ; in the present or living zoology, only 

 two genera of fishes exhibit a similar arrangement of the tegu- 

 mentary skeleton, namely, the lepidosteus of the Ohio, and the 

 polypterus of the Nile. In the sturgeon we find a very complete 

 tegumentary skeleton, but not arranged in the admirable mecha- 

 nical manner of the extinct ganoid fishes." E. K.] 



SECOND PRIMARY DIVISION. 

 THE ANNELIDES OR ENTOMOZOARIA. 



507. The animals composing this division are so distinct 

 from the preceding as to be recognised at a glance. Their 

 bodies, in fact, are composed of segment-like rings, placed in 

 a file behind each other. In some, this cumulated appearance 

 is confined to the existence of a certain number of transverse 

 folds grooving the skin and engirdling the body, but in most 

 the animal is enclosed in a sort of solid armour, composed of 

 a series of rings united to each other, or so articulated as to 

 admit of motion. This armour serves the purpose in some 

 measure of a skeleton, for it determines the general form of 

 the body, protects the soft parts, offers points of attachment 

 for the muscles, and performs the office of levers ; thus, it is 

 often called an external skeleton. But it must not be for- 

 gotten that it is merely the skin hardened and become rigid, 

 D D 



