PLATE XVI. 



Three diagrammatic figures for the comparison of the nerve system of the Nemertea, of the Vertebrata, 

 and of the Cephalochorda. 



Fig. 1. The chief points in the nervous system of the Nemertea. In, hi, the lateral nerve-stems with 

 their anterior swellings, the brain-lobes, Lg. The latter are connected in front by a 

 strong ring-shaped commissure, the former by much thinner but unmistakable 

 commissures, dvr, metamerically placed both dorsally and ventrally in a continuous 

 plexus of nerve-tissue, that ensheathes the body. This plexus is no further indicated in 

 the figure. It moreover carries a longitudinal medio-dorsal nerve-tract m, which is 

 also continued forwards, in front of the brain commissure, with which it is connected. 

 Into this medullary nerve m the transverse tracts dvr converge. From the latter (sensu 

 strictiori from the plexus) radial nerve twigs of sensory and motor significance can be 

 traced both in centripetal and in centrifugal direction, but these are not indicated in the 

 figure. Other similar nerve twigs, indicated by vi.sy, innervate in a similar way the 

 wall of the oesophagus and of the blood-lacuna surrounding it (c/. PI. XIV. figs. 3, 4). 

 The innervation of the oesophageal wall is, moreover, brought about by a paired nerve, 

 v, springing from the brain-lobes, and not strictly separate in its peripheral distribution 

 from that of the nerves vi.sy. en, nerves to the tip of the head ; M, mouth. 



Fig. 2. The chief points in the nervous system of the Vertebrata. 



Instead of the medullary nerve we find here in the corresponding situation the 

 medulla spinalis m and its anterior enlargement, the brain. The spinal nerves are 

 represented in their primitive condition, i.e., with the dorsal (sensory) roots dr, and the 

 ventral (motor) roots vr, not yet connected into a spinal nerve of higher order. 

 Anteriorly is represented the union of more than one metarneric transverse root into a 

 complex polymerous vagus nerve, Vag, motor and sensory branches of whicli are 

 represented as innervating the branchial pouches of the fore gut (the latter not specially 

 indicated in the figure). From the vagus ganglion also springs hi, the ramus lateralis 

 nervi vagi or nervus lateralis ; hi', the left nervus lateralis. This nerve is continued 

 f'jrwards into other ganglionic swellings Lg, representing the series of ganglia of the 

 cephalic nerves. The visceral branches springing from the dorsal roots and inner- 

 vating vegetative organs are indicated by vi.sy : they are supposed not yet to have united 

 into a longitudinal "sympathetic nerve." In front the region innervated by them 

 overlaps and fuses with that of the intestinal branch v. of the vagus, and of other 

 cephalic nerves. M, mouth. 



Fig. 3. The chief points in the nervous system of Amphioxus. 



The medulla spinalis m without any considerable anterior enlargement ; en, pairs of 

 cephalic nerves ; dr, dorsal roots ; vr, ventral roots ; the former springing from the 

 medulla at a higher level than the latter. The dorsal roots are connected together in a 

 plexiform arrangement; they give off visceral branches (vi.sy). M, mouth. 



