ON THE ANATOMY OF AMPIIIOXUS LANCEOLATUS. 387 



traced with the greatest ease to within 1-1 6th of an inch of 

 the extremity of the chorda dorsalis, does not dilate into a 

 brain at all. It may be urged that we ought to consider the 

 anterior half of the middle third of the spinal marrow, where 

 it is most developed, to be the brain, and all that portion of 

 the chorda dorsalis which is in connection with the branchial 

 cavity, as the cranium. That this does not express the true 

 relation of the parts, is evident from the fact, that this portion 

 of the cord, to its very extremity, gives off nerves, which are 

 too numerous to be considered as cerebral, but more especially 

 from the mode of distribution of the first and second pairs, 

 which, in my opinion, proves the anterior pointed extremity 

 to be the representative of the brain of the more highly 

 developed vertebrata. A brain of such simplicity necessarily 

 precludes, on anatomical grounds alone, the existence of organs 

 of vision and of hearing. These special organs, developed in 

 the vertebrata at least, in a direct relation with the cephalic 

 integuments and the brain, could not exist, even in the form 

 of appreciable germs, in the Lancelet. The black spot which 

 Retzius took for the rudiment of an eye may probably have 

 been, what also deceived me at first, a portion of the black 

 mud which floats about in the branchial cavity, and which 

 adheres obstinately to the parts in the neighbourhood of the 

 oral filaments. The first pair of nerves, although very minute, 

 in accordance with the slight development of the parts about 

 the snout, and the want of special organs of sense, might, from 

 their position and relations, be considered as corresponding to 

 the trifacial in the higher vertebrata. The second pair 

 appears to be the vagus, not only from its distribution as a 

 longitudinal filament on each side of the body, as in other 

 fishes, but also from its relations to the hyoid apparatus and 

 branchial cavity, to which division of organs the eighth pair 

 in fishes is specially devoted. The distribution of a branch of 

 this nerve, however, along the base of the dorsal fin, and the 

 course of the posterior part of the main branch, would appear 



