CEPHALOCHOEDA 141 



The nerve cord is made up of nerve cells or neurons and 

 their prolongations or axons, and these are supported by 

 connective tissue. Some of the nerve cells are large and give 

 off large axons, or giant fibres, which run along the cord. 

 The smaller cells are mainly associated with axons which 

 leave the cord in paired bundles, the nerves. Various writers 

 have attempted to homologise the anterior nerves with 

 the nerves of the Craniates. The nerves generally are seg- 

 mental, and occur in an alternate series of independent dorsal 

 and ventral roots. The dorsal arise singly from the cord, 

 the ventral by a fan of separate fibres which supply the muscle 

 fibres of the myotome opposite which each arises. The 

 dorsal roots supply the skin and the transverse muscles of the 

 metapleure. The dorsal roots are therefore like the dorsal 

 roots of the brain of the Craniates in being intersegmental 

 and extra-myotomic in distribution. The ventral roots agree 

 in origin and distribution with the ventral roots of the brain 

 and spinal cord of the Craniates. The dorsal roots are mixed 

 that is, they contain motor and sensory fibres while the 

 ventral are purely motor. 



Anteriorly a pair of nerves arise from near the front end 

 of the brain and are distributed to the proboscis ; a second 

 pair arise dorso-laterally and run to the pigment spot or eye 

 spot. The mouth is supplied by a series of branches derived 

 from the first eight dorsal roots. 



' The excretory organs consist of a series of protonephridia, 

 and it is a remarkable feature of the creature's organisation. 

 The protonephridia are ectodermal in origin, and are developed 

 from the ectoderm of the atrial cavity as that cavity is being 

 formed. They end blindly, and the epithelium of the tube is 

 produced into numerous solenocytes which come into close 

 relationship with the efferent branchials, against the walls of 

 which the solenocytes impinge. They open dorsally into the 

 atrial cavity, and there are about a hundred pairs altogether. 

 They are interesting because they are so different from the 

 kidneys of the higher vertebrates, because they are developed 

 in the pharyngeal region alone, and because nephridia are 

 not developed in the groups below to which Amphioxus is 

 more nearly related (fig. 67). 



