88 



ANATOMY OF AMPHIOXUS. 



WijHE that the inner plexus on both right and left halves 

 of the oral hood is exclusively formed by nerves which 

 arise from the left side of the central nervous system ; 

 and, further, that the nerve-supply of the velum is fur- 

 nished by branches from the fourth, fifth, and sixth dorsal 

 nerves of the left side only. This asymmetrical innerva- 

 tion of the velum and inner (glandular) surface of the 



oral hood will be referred to 

 again after the consideration 

 of the larval development. 



The peripheral ganglionic 

 enlargements which are so 

 characteristic of the two pairs 

 of cranial nerves must be cor- 

 related with the sensibility of 

 the snout. As the nerve-fibres 

 are continued beyond them, 

 they are not to be regarded as 

 end-organs, but simply as peri- 

 pheral ganglia. Their structure 

 is shown in Fig. 44. They 

 were discovered by the great 

 French naturalist Ouatrefages in 1845. Each of them 

 is composed of from one to four nerve-cells, with granular 

 protoplasm and a large nucleus. Each group is enclosed 

 in a sheath which is a continuation of the sheath of the 

 nerve itself. The sheath is lined internally by an endo- 

 thelium. According to Fusari the nerve-fibres enter into 

 direct connexion with the cells, though some would appear 

 to pass round them. 



The peripheral nervous system of Amphioxus can only 

 be compared definitely, at present, in its broadest features 

 with that of the higher Vertebrates. The determination 



Fig. 44. — Peripheral ganglion 

 cells of the cranial nerves of Amphi^ 

 oxus. (After Fusari.) 



