70 LECTURE IV. 



logous but stronger weapon of the Siluroids is usually beset with den- 

 ticles of the same hard substance, sometimes anchylosed to the spine, 

 sometimes movably articulated with it. M. Agassiz has found that the 

 fixed denticles have the same osseous textui^e, characterised by ra- 

 diated corpuscles in concentric layers, as the spine itself; whilst the 

 movable denticles present a simpler structure, being permeated by 

 calcigerous tubes, radiating from a central vascular pulp cavity, like 

 teeth ; but the comparative anatomist who has extended his obser- 

 vations beyond the class of Fishes, will pause before he admits the 

 sweeping conclusion which the celebrated ichthyologist draws from 

 his interesting microscopical observations.* 



The distinction between the internal or sj)lanchnic and external 

 skeletons does not rest upon the microscopic character of their tissues ; 

 if it did, and if every calcified plate or spine that presented the cha- 

 racteristic radiated cells of bone, were to be classed with the pieces of 

 the internal skeleton, we must cease to regard the scales of the Cro- 

 codile, and the tesselated carapace of the Armadillo, as parts of the 

 external or dermal skeleton. 



LECTUKE IV. 



THE SKULL OF FISHES. 



Passing from the trunk to the head, we find in the Lancelet {Bran- 

 chiostoma, xxx.), at the lowest step of the Vertebrate series, that the 

 cranium is not indicated by difference of size or structure of the ru- 

 dimental vertebral column, but consists of that gradually contracting 

 anterior termination of the neural canal, which retains its primitive 

 fibro-membranous wall, (Jig. 46. n), without any superaddition of 

 parts, and is supported by the tapering end of the gelatinous ' chorda 

 dorsalis' (ib. ch). This part, in the Lancelet, even extends farther 

 forwards than the cranial end of the neural canal, indicating the non- 

 development of the prosencephalon and corresponding part of tlie 

 cranial cavity. In fact, thei'e is no ganglionic cerebral expansion 

 whatever in this vermiform fish : the epencephalon or medulla oblon- 

 gata is indicated by the origin of the ti-igeminal nerve (ib. ob), in 

 advance of which the mesencephalic segment sends off" the short optic 

 nerve to the dark ocellus {op), and there terminates, somewhat 

 obtusely, beneath what Dr. KoUiker (xxxu. p. 32.) has described as a 

 ciliated olfactory capsule (ib. ol). The cranium of the Lancelet, 



* Les genres Hypostoma et Callichthys presentent cette singuliere structure, et 

 prouvent par la mcme que les difFerences qu'on a voulu etablir, entrc un sijuclette 

 paucicT ou extorue, et un squelette interieur ou intestinal, sont denuees de tout 

 fondement." — Puiisons FossUes, torn. iii. p. 213. 



