PERIPHERAL NERVOUS SYSTEM IN NEMERTINES. 487 
enclosed between two muscular layers, as it was described 
for the Schizonemerteans, and, moreover, corresponding with 
the latter in the fact that this layer does not occur also in 
the head in front of the brain, as it partially does in Cari~ 
nella. In all genera higher differentiated than the latter 
this plexus-like arrangement in the head (which I first 
mistook for a considerable number of parallel nerves, on 
account of the appearance in transverse sections) appears 
to have developed into a smaller number of distinct nerve- 
stems, dichotomously dividing and innervating the tip of 
the snout, the muscles of the head, and the eyes, when 
present. 
The fact of the presence of the nerve-sheath in Carinella 
on the outside of the muscular body-wall and under the 
immediate covering of the ectoderm is of the more impor- 
tance, as we have indeed to look upon this genus as one of 
the less differentiated of all Nemerteans^ which must serve 
as a starting-point for comparisons by which to determine 
the morphological significance of any structure which is 
common to this and other genera. 
In the preparation figured (PI. XXXII, fig. 4) I find that 
processes {np.) are being sent out from this layer as they were 
described for the Schizonemertini. Here I only succeeded, 
however, in demonstrating such as were turned towards 
the muscular layers ; in other sections I find that processes 
towards the epidermic cells are also present (fig. 5). As 
the nervous layer is generally closely applied to the muscles, 
it is difficult to observe the centripetal processes, and only 
in such a case as the one figured, where by some reason or 
other this layer lies somewhat removed from the subjacent 
muscles, can these important ramifications be noticed. 
The diagrammatic figures I, 2 and 3, will now be more 
intelligible after this rapid exposition of the facts. Fig. I 
applies to Carinella. The lateral trunks and the nervous 
layer are wholly outside the muscular body-wall, processes 
from this layer being sent outwards and inwards. Fig. % 
stands for Valencinia, Folia, and all the Schizonemertini. 
The trunks and the layer are enclosed loithin the muscles, the 
peripheral processes are sent out in both directions. Fig. 3 
finally represents the Hoplonemertini, a stage of higher 
development, in which the continuous cellular layer uniting 
the longitudinal trunks and ensheathing the whole body has 
become localised and differentiated into separate nerve- 
branches springing from these trunks at regular intervals. 
' Cf. A. A. W. Hubrecht, “The Genera of European Nenierteans 
critically revised,” ‘Notes from the Leyden Museum/ vol. i, p, 19. 
