134 THE VOYAGE OF H.M.S. CHALLENGER. 



which was first expressed in a footnote to a former paper of mine (X, p. 438). There 

 can hardly be any doubt as to the existence, consequent upon natural selection, of a 

 constant tendency in the different component parts of living organisms towards simplifi- 

 cation and increased efficiency (Eoux's Kampf der Theile im Organismus). This fact 

 enables us to understand the gradual supremacy of the median cord in the Nemertean 

 plexus over the two lateral ones. It seems as if it were mathematically demonstrable that 

 for the delicate adjustment of the impressions from the exterior to the co-ordinated 

 movements thereby occasioned, one longitudinal central stem in bilateral, lengthened 

 animals, would be more efficacious than two lateral ones. And if we ask if, at the final 

 stage of this struggle for supremacy between three longitudinal stems, any remnants of 

 the lateral cords are yet detectable in the Vertebrate embryos, perhaps even in the adults, 

 T am inclined to answer in the affirmative. Here I must be allowed to insert a reference 

 to the three figures on PI. XVI., which will facilitate the exposition of the further conse- 

 quences of the hypothesis I am here developing. Fig. 1 represents the chief points in the 

 nervous system of the Nemertea. The brain-lobes are simple lateral swellings of the 

 longitudinal stems, as in Carinella ; plexus, medulla and transverse stems, together with 

 brain-lobes and lateral stems, may be considered as forming part of the integument [cf. 

 Carinina). A double innervation of the respiratory portion of the intestine is indicated ; 

 one due to visceral branches {vi.sy) springing from the plexus (or from its transverse 

 tracts), the other to the more specialised nerve (v), which has above been indicated as 

 the Nemertean vagus nerve. The plexus and its innumerable radial fibres, both sensory 

 and motor, are not indicated in this figure, nor are the nerve-stems which, when present 

 (PL XIV. fig. 2), pass from the lateral stems directly to the integument. 



This figure must now be compared with the two others. Of these, PL XVI. fig. 2, 

 diagrammatically represents the chief points that may be considered as characteristic 

 of the nervous system of a lower Vertebrate, in which the dorsal and ventral roots of the 

 spinal nerves {dr and w) are as yet separate nerve-tracts, in which the sympathetic nerve 

 system is as yet only represented by visceral branches given oft' by these dorsal roots 

 (vi.sy), and in which the polymerous character of a primitive vagus ( Vag) is established. 



PL XVI. fig. 3, stands for Aynj>hioxus, as far as we know its nervous system, more 

 particularly through the researches of Rohon and others. It diSers from the foregoing by 

 the absence of a distinct brain swelling, and of a polymerous vagus. A number of spinal 

 nerves are considered as homologues to the vagus of Vertebrates by Eohon. The 

 commissural connections between the successive spinal nerves form a plexus, which is 

 peripherally even much more elaborate, according to Rohon's figures. This plexus does 

 not reveal the presence of any distinct lateral longitudinal nerve, nor any ganglia of 

 spinal or cephalic nerves. The latter (en) may be said to be three in number. Visceral 

 branches (vi.sy) are given oft" by the dorsal nerves (dr). The ventral ones, springing from 

 the lower edge of the medulla, are here represented as shorter stems (vr). 



