INTERNAL ANATOMY. 89 



of the particular homologies in the two cases forms one of 

 the most difficult problems of comparative morphology. In 

 correlation with the low grade of cephalisation to which 

 Amphioxus has attained, there are only two pairs of 

 cranial nerves, the succeeding nerves retaining their 

 primitive spinal character. The dorsal spinal nerves, 

 furthermore, possess features which are specially charac- 

 teristic of the cranial nerves of the Craniota. Such are 

 their mixed sensory and motor functions, and the position 

 of their dorsal and ventral branches ectad of the muscula- 

 ture. As already indicated above, the walls of the gill-slits 

 of the craniate Vertebrates are innervated by cranial 

 nerves, while in Amphioxus this is done by spinal nerves. 

 (Cf. Fig. 92 ; see also below, p. 163.) 



In transverse section the spinal cord of Amphioxus is 

 seen to have somewhat of a triangular shape. The central 

 canal has the form of a vertically elongated split, commenc- 

 ing from the vertex of the triangle, and extending two- 

 thirds of the way downwards into the cord. For the 

 greater part of its extent, however, the two sides of the 

 canal are closely approximated together so as to obliterate 

 the lumen, which widens out again below, and presents the 

 appearance of a circular or oval tube. The sides of the 

 canal are lined by an epithelium the cells of which, starting 

 from an indifferent condition in the embryo, have become 

 modified in several different directions. Some are ganglion- 

 cells, and others send out long radial processes which trav- 

 erse the substance of the nerve-cord, and serve to hold it 

 together. These are the supporting fibres (Fig. 43). The 

 cells in the nerve-cord form a much smaller proportion of 

 the bulk of it than the nerve-fibres do. The latter run 

 mostly in a longitudinal direction, and produce a punctate 

 appearance in cross-section. 



