86 ANATOMY OF AMPHJOXUS. 



cranial nerves of the Craniota so far resemble the dorsal 

 spinal nerves of Amphioxus that they run external to or 

 ectad of the somites of the head. 



The ramus dorsalis of a spinal nerve breaks up into a 

 number of finer nerves, which supply the skin of the back. 

 The ramus ventralis similarly gives rise to a number of 

 cutaneous nerves, but in addition it gives off a branch 

 which passes inwards below the longitudinal muscles of 

 the body-wall, between them and the transverse muscles 

 which lie in the floor of the atrium. This is the visceral 

 branch of the spinal nerve. The visceral nerves innervate 

 the transverse muscles and form an elaborate plexus on 

 the surface of them.* 



Thus the dorsal spinal nerves of Amphioxus are of a 

 mixed nature, sensory and motor, but chiefly sensor}-. 



The ventral roots are entirely motor. On their emer- 

 gence from the spinal cord they spread out like a fan 

 and terminate upon the muscle-fibres of the myotomes 

 (Fig. 43)-** 



The muscles which are not innervated by the ventral 

 spinal nerves are the transverse or sitbatrial muscles, the 

 muscles of the month (velum), and oral hood, and probably 

 the anal sphincter. These are supplied by the so-called 

 visceral branches of the dorsal nerves. The nerve-supply 

 of the oral hood is illustrated in I^ig. 42. It arises from 

 branches of the third to the seventh dorsal nerves. These 

 branches are distributed in two different ways : one set 



* The visceral nerves also send up branches, which pass up through the 

 ligamentum denticulatum to the wall of the pharvnx. (Fi's.^RI; see below, 

 p. .) Here they form the branchial plexus described bv RoHON, who 

 thought these nerves contained elements of the J'agus of the Craniota. 

 The portions of the visceral nerves innervating the transverse muscles (these 

 branches being discovered by Rolfh) were held by RoHON to contain 

 elements of the Sytnpatlutic syskiii of Craniota. 



