ALIMENTARY CANAL. 



19 



The ventral nerves (Fig. 7) are not united 

 in bundles nor surrounded by a sheath, but 

 issue as linear groups of fine fibres, which pass 

 immediately to the adjacent myotome. They 

 arise slightly anterior to the dorsal nerve of 

 the segment to which they belong, and are 

 exclusively motor for the muscles of the myo- 

 tomes. 



Sense organs, Scattered amongst the ecto- 

 derm cells are cells bearing short hairs. They 

 are specially numerous at the front end of the 

 body and round the mouth. They may be 

 supposed to be tactile organs. They are also 

 found in the mouth and on the velum. 

 Organs which are supposed to be visual and 

 olfactory have already been described. 



The most striking peculiarities of the ner- 

 vous system of Amphioxus as compared with 

 that of the Vertebrata are the absence of an 

 anterior cerebral enlargement ; the peculiar form 

 of the central canal, the dorsal fissure-like por- 

 tion of which is probably represented by the FIG. ?. Anterior en i 

 dorsal fissure of the vertebrate spinal cord ; 

 the absence of a junction between the dorsal 

 and ventral nerve roots and of a ganglion on 

 the dorsal roots ; and, lastly, the imperfect 

 condition of the organs of special sense. 



IV 



is a 



ches of the second 

 nerve arises indepen- 

 dently from the cord 

 in the preparation 



after 



ys f ' a n c 2 " 

 It leads into a spacious fJ n (J sec z A / d / n 



The alimentary canal. The mouth 

 large, somewhat circular opening, placed on 

 the ventral side of the body, a little distance t s c r h y ne 

 from the front end. 



cavity, the buccal cavity, which is bounded by 

 a fold of the integument called the oral hood 

 (Fig. 4). The free edge of the oral hood, 

 which may be called the lip, contains the skeletal framework 

 already described, and bears a number (from twenty to thirty, 

 increasing with age) of delicate ciliated processes, the oral 

 cirri. The right side of this oral hood is, as has already been 

 mentioned, continuous with the preoral ventral part of the 

 median fin, which is in accordance with what might be expected 



