48 



ZOOLOGY 



SECT. 



tube. The anterior end of the neuron, containing these two 

 cavities, is to be looked upon as the brain, although not dis- 

 tinguishable externally from the remaining portion or spinal 

 cord. 



The anterior and dorsal region of the brain is produced into 

 a small hollow pointed pouch which comes into relation with the 

 olfactory organ and is called the median olfactory lobe. In its 

 posterior and ventral region a depression has been described 

 which appears to correspond with the infundibulum of the 

 Craniata (vide infra). 



The neuron is mainly composed of longitudinal nerve-fibres with 

 abundant nerve cells mostly grouped around the neuroccele. At 



D 



FIG. 702. Amphioxus lanceolatus. A, brain and cerebral nerves of a young speci- 

 men ; B, transverse section through neuropore ; C, behind cerebral ventricle; D, through 

 dorsal dilatation, ck. notochord ; cr. cerebral ventricle ; dil. dorsal dilatation ; e. eye-spot ; 

 np. neuropore ; olf. olfactory pit ; /, //, cerebral nerves. (From Willey, after Hatschek.) 



intervals giant nerve-cells occur, multipolar cells of immense 

 proportional size, connected with nerve-fibres of unusual size, the 

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

 of Chsetopods (Vol. I. p. 438) which, however, have no nervous 

 function and are 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. 702, 1. 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 



