So Evolution and Distribution of Fishes 



teans, and that run backward on each side of the body to 

 form a union above the intestine; (3) the two dorsal 

 nerve threads which — more or less separate and important 

 in rhabdocoels — have fused lengthwise except at their roots 

 and are only moderately strong. 



In transition to the vertebrates the following seems 

 to have taken place: (a) the strong lateral nerves, that 

 in metanemerteans have already formed a commissure 

 above the intestine, have gradually risen upward dorsally, 

 from behind forward, till apposition and lateral union of 

 the two nerves have been effected forward to their root 

 origin from the ganglia; (b) during this process the dor- 

 sal paired nerve has by degrees become surrounded by 

 the uprising lateral nerves, and its important optico-motor 

 functions have by degrees been usurped by the laterals, 

 so that it suffers marked reduction and degeneration; (c) 

 the ventral pair of nerves or "mundschlund" system of 

 Burger "from its origin along the lower posterior edges 

 of the ventral brain-masses, and its abundant distribution 

 round the mouth, the pharynx, the alimentary canal, and 

 the skin, would correspond with the ventral nerve threads of 

 Turbellarians. This seems gradually to have evolved into 

 what might be termed the ventro-sympathetic in proto- 

 chordates, and later into the sympathetic system of cyclo- 

 stomes and higher vertebrates." (7:440). 



As indicative of such gradual evolutionary changes it 

 is noteworthy that in Amphioxiis {Branchiostoma) and in 

 true fishes, the first rudiment embryologically of the spinal 

 cord appears as an epiblastic groove, that spreads forward 

 from the anal region, and on either side of which arises 

 a plate or thickening of tissue lengthwise that we would 

 interpret as the lateral nerves of metanemerteans. By 

 gradual uprising, thickening, and fusing of the upper edges 

 of these plates the neural canal is formed. As the two 

 lateral plates or nerves continue to thicken they so press 

 on the canal as to convert this into a median slit. But in 

 Amphioxiis the complete dorsal fusion of the plates or 

 nerves anteriorly is not effected, so that a dorsal "longi- 

 tudinal cleft" is left. It should further be noted that in 

 metanemerteans a copious anastomosis of nerve-fibres takes 



