Anatomy and Histology of Nemer tines. 403 
Tetrastemma aquarum dulcium , that the excretory vessels of 
Nemertines are provided with flame-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 Lanice conchilega , that remarkable Tere- 
bellid in which four nephridia are 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. In 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 MHntosh, Hubrecht, and Oudemans, that this 
occurs in all forms belonging to Group IT. 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 Carinoma 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 affinities thus : — 
* Hubrecht, op. cit. tab. xi. 
29 * 
