NERVOUS SYSTEM OF AMPHIOXUS 



d.n. 



37 



v.n. 



Fig. 13a. Nerve-cord of amphioxus. 

 d.n. dorsal nerve-root; g. 'giant' nerve-fibres; v.n. ventral nerve-root. (After Retzius.) 



Ret 



rec 



v.m.c 



s.m c 



median giant fibre 



Fig. 13ft. Stereogram illustrating the structure of the spinal cord in an adult amphioxus. 



The receptor system is made up of a more or less continuous column of bipolar cells of 

 Retzius (Ret.), together with smaller cells of various types (rec). According to Johnston 

 these receptor cells (1,2 and 3) can be regarded as equivalent to the dorsal root ganglion 

 cells of vertebrates. The other type of receptor cell is the giant Rohde cell (Roh.), which has 

 a large axon and elaborate dendritic system. It is probable that at least some of these cells 

 possess a peripheral axon running in the dorsal root. I.e. longitudinal connective cell. 



The visceral motor cells (v.m.c.) are arranged segmentally, one per segment. 



The somatic motor cells (s.m.c.) lie at a different level in the cord from the ventral 

 roots. 



Other cells in the cord are internuncials of various types. (After Bone.) 



The fibres of the peripheral nerves differ from those of vertebrates 

 in that they have no thick myelin sheath that will blacken with 

 osmium tetroxide. The nerve trunks are surrounded by an epineurium 

 with connective tissue cells but there seem to be no Schwann cells 

 accompanying the nerve-fibres (Bone, 1958). 



