434 
DR. A. A. W. HUBRECHT. 
follow each other at regular intervals, probably corresponding 
in number with the successive internal metameric divisions 
of the body. They remain undivided for a certain distance, 
and, spreading out in the muscular tissue, are quickly sub- 
jected to a dichotomic division, which gives rise to numerous 
smaller twigs. They can be clearly made out by compres- 
sion of the living animal, with or without the use of dilute 
acetic acid, as well as in longitudinal and horizontal sec- 
tions. I generally noticed two branches springing from the 
same region of the longitudinal trunk, one turning to the 
dorsal, the other to the ventral half of the muscular body- 
wall (see PI. XXXII, fig. 8, and in another paper, ^ Aan- 
teekeningen over de Anatomie enz. van eenige Nemertinen,^ 
Utrecht, 1874, pi. i, fig. 2). The elaborate plexus of 
branches observed by Macintosh is worthy of note, as I 
will try to show hereafter ; it was sometimes observed by 
myself, although in a lesser degree than is represented in 
Macintosh^ s figure. 
The two other Nemertean suborders, the Paljeo- and the 
ScHizoNEMERTiNi, of which I examined several hundreds of 
sections, most of them forming uninterrupted series, gradually 
puzzled me more and more by the absence in all these sec- 
tions, longitudinal as well as transverse, of any small branch 
springing from the longitudinal marrow-trunks. Even the 
faint traces which Macintosh believes to have observed now 
and then did not make their appearance, and the better the 
sections were preserved and stained the more conspicuous 
was their absence. On the other hand, well-preserved sec- 
tions of Schizonemerteans always show the thick coating of 
nerve-cells surrounding the central, more fibrous, tissue of 
the longitudinal trunks very clearly (PI. XXXII, fig. 6). 
The whole of the trunk and cellular coating is again sur- 
rounded by another layer of homogeneous connective tissue, 
which separates at the same time the external layer of longi- 
tudinal muscular fibres from the internal circular muscular 
layer, not only in the vicinity of the nerve-trunks, but 
throughout the whole extent of the body (cf. PI. XXIII, 
fig. 12, in the July number of this Journal). In this homo- 
geneous layer is situated another somewhat thinner layer of 
tissue of an entirely different histological character. 
ft is this last-named layer which we now have to ex- 
amine in detail. Staining reagents, as picro-carmine, &c., 
have very little effect upon the bulk of the tissue, although 
they render conspicuous in it the presence of numerous 
nuclei, which distinctly stand out with a more or less deep red 
tinge, hiach nucleus is provided with a nucleolus, and now 
