136 VOYAGE OF H.M.S. CHALLENGER. 



have established for the lower Chordata (Cephalochorda and Cyclostomata) that the 

 typical chordate spinal nerve is not originally provided with a double root, but that tliis 

 double root appears to have arisen by the coalescence of what were primitively in the 

 groups just mentioned, separate and alternating dorsal and ventral nerve-tracts. 

 With these so much simpler spinal nerves the transverse nerve-stems of the Nemertea 

 undoubtedly offer points of comparison. These Nemertean nerves specially differ from 

 the Vertebrate spinal nerves in two respects : (l) they give off nerve-fil)res in different 

 directions, which are j^robably motor as well as sensory and visceral, according to the 

 different organ systems they terminate in ; and (2) they go round ventrally, each of them 

 forming a loop all round the body. As to the fii'st point of difference just alluded to, it is 

 the expression of a low and primitive degree of differentiation, and when a step forwards 

 is made, differentiation of labour will tend to develop certain tracts more particularly con- 

 taining sensory and visceral nerve-fibres, which are more especially dii-ected towards 

 the epithelia (the primitive dorsal or posterior roots), and others more particularly 

 containing motor nerve-fibres, and more especially directed inwards towards the muscles 

 (the primitive ventral or anterior roots), because the musculature, as was already 

 mentioned, is originally situated internally to the nervous system. 



For the present we can only hold it to be estalilished that the fibres of these three 

 categories are blended in the Nemertean plexus, without being able to determine in 

 how far the specialisation therein observed, of the appearance of transverse metameric 

 nerves, may at the same time be accompanied by a commencement of differentiation, such 

 as has just been alluded to. We may, in other words, not yet assume that among these 

 metameric stems there is already a tendency to an alternation between such as have 

 sensory and visceral, and such as have motor predispositions. 



Only in a few cases may we be justified in saying that certain nerve-tracts belonging 

 to the Nemertean peripheral S3'stem are more especially sensory (PI. XIV. fig. 2) or 

 visceral (PI. XIV. fig. 4), and these no doubt offer important analogies in their situation 

 and connections to similar nerve-tracts of the Vertebrata. 



The second point of difference, viz., the continuity in the ventral median line of the 

 transverse tracts of the Nemertea, is no doubt a consequence (a) of their origin in a 

 perfectly continuous plexus, (b) of the cylindrical arrangement of the muscular layers, 

 which in most cases are uninterrupted both in the dorsal and in the ventral median line. 

 It is all the more important to notice, that more especially in the primitive Carinellidaj, 

 the tendency is very marked towards a scission of this muscular body-wall into a right 

 and a left half (cf. p. 72). A comparison of figs. 4, 5 on PI. XL will show this. 

 In the Schizonemertea, too, and in Eupolia it affects the primiti\'e muscular layer (PI. 

 XI. figs. 10, 12), in a more or less marked degree. 



This longitudinal scission is no doiibt the first expression of the phenomenon which 

 shows us the musculature of the right and left half of the body, developing quite inde- 



