xm 



PHYLUM CHORDATA 



53 



intervals giant nerve-cells occur multipolar cells of immense 

 proportional size, connected with nerve-fibres of unusual thickness 

 the giant fibres. The latter" appear to correspond with the giant 

 fibres of Chaetopods (Vol., I. p. 466), which have sometimes been 

 supposed to have no nervous function and to be mere supporting 

 structures. 



The peripheral nervous system consists of the nerves given off 

 from the neuron. They are divisible into two sets, the first 

 consisting of two pairs of cerebral nerves (Fig. 

 756, I. and! II.) arising from the brain, the 

 second of a large number of spinal nerves 

 arising from the spinal cord. The cerebral 

 nerves take their origin in front of the first 

 myomere, the first from the anterior extremity 

 of the brain, the second from its dorsal region : 

 they are both distributed to the snout, their 

 branches being provided towards their extremi- 

 ties with numerous ganglia containing nerve- 

 cells. The spinal nerves are segmentally 

 arranged, and, in correspondence with the dis- 

 position of the myomeres, those of the right 

 and left sides arise alternately, and not opposite 

 one another (Fig. 757). In each segment there 

 are two nerves on each side, a dorsal nerve, 

 arising by a single root from the dorsal aspect 

 of the spinal cord, and a ventral nerve, arising 

 by numerous separate fibres : the dorsal nerves 

 supply the skin and the transverse muscles and 

 are therefore both sensory and motor, the ven- 

 tral nerves are purely motor, supplying the 

 myomeres. 



Sensory Organs. At the level of the an- 

 terior end of the brain is a narrow ciliated de- 

 pression, the olfactory pit (Fig. 756, olf.), 

 opening externally on the left side of the 



T ', -, ., i j ',-, 



snout and connected at its lower end with 



the median olfactory lobe. This structure is 



supposed to be an organ of smell : in the larva 



its cavity is in direct communication with the neuroccele through 



the neuropore (np.). 



ALL unpaired pigment-spot (e) in the front wall of the brain is 

 usually referred to as a median cerebral eye. There is no lens or 

 other accessory apparatus, and experimental evidence seems to 

 show that this so-called eye is not sensitive to light. Smaller 

 eye-like organs with or without pigment occur in the spinal cord 

 throughout the greater part of its length. There is no trace of 

 auditory organs. The groove of Hatschek, on the roof of the buccal 



lanceolatus. An- 

 terior portion of neuron 

 from above, showing 

 Willey, 



