ALIMENTARY CANAL. 



19 



ol..- 



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. Tliey 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 Vertehrata 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 fiq. 7.— Anterior en i 

 dorsal fissure of the vertebrate spinal cord ; ?4,„pAlox«r^s*j?owin" 

 the absence of a junction between the dorsal fljstlwrplt^s^^ni ^^2^ 

 and ventral nerve roots and of a ganglion on anrp"Lse'o*p''potitro°il^ 

 the dorsal roots ; and, lastly, the imperfect ^^^^^^'^^^ o?'ihe'brln- 

 condition of the organs of special sense. ^^^^.^ "arises^indepTn'^ 



The alimentary canal. The mouth is a ;n"-th/™preS-at^'n 

 large, somewhat circular opening, placed on in'l'^„^g'°made^ uftlr 

 the ventral side of the body, a little distance ti'y^i>^ole,,fnf% 

 from the front end. It leads into a spacious S'iail f /!°//T/F 

 cavity, the buccal cavity, which is bounded by ieKt'^onr'^miTi *o^f 

 a fold of the integument called the oral hood ro'tTohhe^saJI"*''''' 

 (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 



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