54 



ZOOLOGY 



SECT, 



delicate connective-tissue sheath which invests the whole nerve- 

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

 cavities, is to be looked upon as the brain, altiiough 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 



Fig. 747.— Amphioxus lanceolatUB. A, brain and cerebral nerves of a young' speci- 

 men : B, transverse section through neuroi_iore ; C, behind cerebral ventricle^ U, through 

 dorsal dilatation, ck. notochord ; <.■<•. cerebral ventricle ; Od. dorsal dilatation ; e. eye-spot ; 

 ■ivp. neuropore ; o\f. 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 thickness 

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

 fibres of Chaitopods (Vol. I. p. 477) 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. 747, 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 

 origin in front of the first myomere, the first from the anterior 



