388 ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 



to show that this nerve, which I have provisionally de- 

 nominated the Vagus, is, in fact, the trifacial, which, in the 

 higher fishes, is not only distributed to all the fins, but holds 

 exactly the same relations to the dorsal and anal fins, and to 

 the spinal nerves, as the nerve now under consideration in 

 the Lancelet. 



The peculiarities in the structure of the spinal cord are 

 not less remarkable than those of its configuration. It is 

 difficult to understand, according to the received opinions on 

 the subject, how a spinal cord destitute of primitive fibres or 

 tubes, and composed altogether of isolated cells, arranged in a 

 linear direction only towards the middle of the cord, can 

 transmit influences in any given direction ; and more especially 

 how the tract of black or grey matter, if it exercises any 

 peculiar function (excito-motary) communicates with the 

 origin of the nerves. The nerves, also, are remarkable, origi- 

 nating in single roots, and containing in their composition one 

 kind only of primitive fibres (cylindrical). 



In reference to the skeleton of the Lancelet, it is evidently 

 of the simplest kind. If we limit the term skeleton to the 

 Neuro-Skeleton, this animal possesses only the primitive form 

 of such a skeleton — a chorda dorsalis without any cranial 

 enlargement, with a dorsal and ventral series of germs of 

 interspinous bones and fin rays — peripheral elements of a 

 spinal column. 



From a consideration of the particular class of embryonic 

 forms to which this fish corresponds, we could not expect 

 either bone or cartilage in the composition of its skeleton. 

 Accordingly, the skeleton consists of a series of sacs, assuming 

 particular forms according to their several positions : flattened 

 in the chorda dorsalis, cylindrical in the fin bones. These 

 sacs are easily derived, according to established histological 

 laws, from the primitive nucleated cells which constitute the 

 tissue of their representatives in the embryo, and contain, in 

 their interior, cells, or the nuclei of cells. This view of the 



