THE CEPHALOCORDA 189 



sensory and motor, are not united to form a mixed nerve, 

 as is the case in higher vertebrates, but proceed straight to 

 their destinations without ever becoming connected with one 

 another. The ventral roots, on emerging from the sheath of 

 the nerve cord, spread out and terminate in the muscle fibres 

 of the myotome to which they belong. The dorsal roots run 

 outwards through the myotomes and divide into dorsal and 

 ventral branches, which run upwards and downwards in the 

 cutis below the epidermis, and break up into a number of 

 fine branches, supplying the epidermis, and also, it is said, 

 the muscles of the floor of the atrium and the pharynx. 



The two first pairs of nerves have no ventral roots, do not 

 supply muscles, and appear to be exclusively sensory in 

 function. Hence they are frequently distinguished as cranial 

 nerves, but, as Amphioxus has no cranium, the name is hardly 

 satisfactory. The first pair arises symmetrically from the lower 

 side of the front end of the nerve cord and runs forward, 

 dividing into a number of branches which are distributed to 

 the fin-like expansion forming the anterior end of the animal. 

 The second pair of nerves also arises symmetrically from the 

 nerve cord, but more dorsally than the anterior pair. Its 

 branches are also distributed to the snout, and both they and 

 the branches of the first pair enter into curious peripheral 

 ganghonic enlargements before they are finally distributed to 

 the epidermis. The bulk of the nerve cord is made up of 

 longitudinal fibres whose cut ends appear as dots in a trans- 

 verse section. Round the central canal and dorsal fissure are 

 nerve-ganglion cells and peculiar fibrous " supporting cells." 

 There are also very large ganglion cells scattered along the 

 centre of the cord from which very large " giant fibres " proceed, 

 running longitudinally down the cord. But the minute struc- 

 ture of the nervous system is too complicated to be described 

 in an elementary treatise. 



The form and position of the gonads has already been 

 described. The cavities of the twenty-six pairs of gonadic 

 sacs are portions of that section of the embryonic ccelom 

 known as the myocoele (see p. 199), which have been cut off 

 from the rest of the coelom so as to form a number of vesicles 

 projecting into the atrial cavity. The sexes in Amphioxus are 

 separate. In the breeding season the gonads of the males and 

 females become hugely distended by the genital products and 



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