628 A. A. W. HUBRECHT. 



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

 forming a loop all round the body. As to the first 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 containing seasory and visceral nerve- 

 fibres, which are more especially directed 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 men- 

 tioned, is originally situated internally to the nervous system. 



For the present we can only hold it to be established that 

 the fibres of these three categories are blended in the Nemer- 

 tean plexus, without being able to determine in how far the 

 specialisation therein observed, of the api)earance 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 

 system are more especially sensory or visceral, and these no 

 doubt offer important analogies in their situation and connec- 

 tions 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 («) 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 

 Carinellidse, the tendency is very marked towards a scission of 

 this muscular body-wall into a right and a left half. 



This longitudinal scission is no doubt the first expression of 



