Anatomy and Ih'stologif of Xe me r tines. 40.3 



Tetrastemma aqnarum dulcium, that tlic excretory vessels ot" 

 Nemertines are provided with Hame-cells. 



Yet we are by no means debarred from a comparison with 

 the Annelids, even as regards the nephridial system, if we 

 bethink ourselves of La/j/ce conckilega, that remarkable Tere- 

 bellid in which four nephridia arc united together on each 

 side by a longitudinal vessel. In this connexion it is of the 

 utmost importance to ascertain whether the forms possessing 

 a number of excretory channels exhibit a metameric arrange- 

 ment of the nephridiopores. Iii all probability the peculiar 

 line of development followed by the excretory apparatus of 

 the Annelids has been influenced by the large size of the 

 body-cavity found in these forms. 



As regards the nervous system, if we start from the lowest 

 forms of the first group and continue our investigations 

 through the other two, we meet with unmistakable evidence 

 of a progressive development ; and this not only in the primi- 

 tive or more complicated composition of the nervous system 

 itself, but also in its varying position, which passes from the 

 epithelial, as described by Hubrecht for the nervous system of 

 Carinina, through the intermuscular stage, until finally we 

 find the nervous system lying entirely within the muscle- 

 layers (infra-muscular). According to Hubrecht the most 

 widely different stages in the progressive passage of the 

 nervous system from the exterior towards the interior of the 

 body is found in representatives of Group I. What is in all 

 probability to a certain extent a resting-stage is reached when 

 we find the nervous system situated outside the circular 

 muscle-layer, but lying immediately upon it. I gather from 

 the works of M'Intosh, Hubrecht, and Oudemans, that this 

 occurs in all forms belonging to Group II. But a transition 

 from this position to the infra-muscular one found in the 

 Enopla is not known in this group. In order to trace this 

 transition we have, indeed, to go back to Group I., and, 

 according to the description and figure given by Hubrecht *, 

 we find it in Garinoma and Cephalothrix. It is therefore 

 from these forms, judging by the position of the lateral nerve- 

 cords, that the Enopla are to be derived; but the genera of 

 the second group can only have sprung from a form in which 

 the lateral cords are still outside the circular muscle-layer. 

 We may therefore represent the aflinities thus : — 



* Hubrecht, op. cU. tab. xi. 



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