18 PHYLUM CEPHALOCHORDA. 



Anteriorly, though the cord itself tapers slightly, the central 

 canal widens out into a spacious vesicle, the cerebral vesicle. 

 This is the only representative of the brain of the Vertebrata. 

 It gives off from its front dorsal wall a small hollow diverticulum, 

 which ends blindly against the inner side of a small asymmetrical 

 pit of ectoderm called the olfactory pit. This pit is placed 

 on the left side of the body, is lined by ciliated cells, and is 

 supposed to be olfactory in function. In the larva the process 

 of the cerebral vesicle and the pit are in communication by a 

 pore, which appears to be the persistent neuropore of the 

 embryo. The cerebral vesicle is also said to possess a ventral 

 diverticulum in the hinder part of its floor, which has been com- 

 pared with the infundibulum of the vertebrate brain, but it is 

 doubtful if it is always present. Posteriorly the cord tapers 

 considerably and ends, usually in a small swelling, just short 

 of the hind end of the notochord. In some cases it is said to 

 extend as a filament round the hind end of the notochord on 

 to the ventral surface. 



The nerves issuing from this cord, except in the case of 

 the first two, are dorsal and ventral in their origin and 

 correspond with the dorsal and ventral roots of the spinal 

 nerves of the Vertebrata. In Am/phioxus they do not join 

 and there are no ganglia on the dorsal roots. The dorsal 

 nerves arise from the dorsal part of the cord. The nerves 

 of the first two pairs (so-called cranial nerves) arise opposite 

 one another, altogether in front of the myotomes. They are 

 entirely sensory, and supply the preoral part of the body. Their 

 finer branches possess nerve cells not far from their terminations. 

 The remainder of the dorsal nerves alternate in their origin 

 on the two sides, and contain motor as well as sensory fibres. 

 They pass out behind the myotome to which they belong, and 

 divide in the subcutaneous connective tissue into dorsal and 

 ventral branches. The ventral of these supplies nerves to the 

 transverse muscle of the sub-atrial floor. The dorsal nerves 

 also supply the muscles of the lips and velum. The sides of 

 the mouth (oral hood) and the cirri are supplied by the third 

 to seventh dorsal nerves, and the nerves to the inner side of 

 the lips of both sides are derived from the dorsal nerves which 

 arise on the left side of the cord (reminiscence of larval con- 

 dition, see below). The velar supply comes from dorsal nerves 

 four to seven. 



