Haswett— aA Monograph of the Temnocephalee. 119 
The nerve-cells (PI. x1. figs. 4-9) appear as swellings in the course or at the end of 
nerve-tubes. Each contains a spherical or subspherical nucleus, ‘015 mm. in diameter 
in 7. fasctata. This always contains a relatively small nucleolus in which all the 
chromatin appears to be condensed; and a delicate achromatin network with irregular 
collections of minute particles similar to those in the nuclei of the excretory cells. 
In the cytoplasm are numerous very fine fibrils mostly arranged concentrically around 
the nucleus, but some radiating outwards from it. 
The brain (Pl. x1. figs. 5 to 9) is situated just in front of the pharynx, and not 
far from the dorsal surface of the body. It consists of a central fibrous mass and 
groups of ganglion-cells. When the former is looked at from above it appears 
squarish ; but it is best described as a broad thick band (PI. x1. fig. 5) which is 
transverse and horizontal in its middle part, but laterally bends downwards and 
backwards to the point where it passes into the posterior nerve-cords. It is the 
transverse part which gives the appearance of squareness.* 
Webert describes the brain in 7. Semferi as consisting on each side of a ganglion, 
am 
with a core of “ Punkt-substanz” surrounded by fibres—the ganglia united by a 
commissure ; but there is no division by a constriction into two lateral portions 
in any of the species I have examined. 
The central part of the brain consists of a finely fibrous material, having the 
transverse commissural nerve-tubes closely applied to it, and sometimes traversing 
its substance, with a number of larger tubes and spaces containing matter similar 
to that which occurs in the interior of the nerve-tubes. On either side are two 
great groups of bipolar, with a few unipolar ganglion-cells ; these are very numerous 
—some hundreds in number altogether—and present a totally different appearance 
from that represented by Weber. One of the groups of ganglion-cells is the root 
of the nerves that pass forwards; every nerve-fibre has near its origin a bipolar 
cell—one process being the nerve-fibre, the other passing into the central substance 
of the ganglion, through which it is frequently traceable. The other group is the 
root of the nerves that pass backwards ; and the relations of the cells to the fibres 
are here the same. The commissural fibres also have ganglion-cells in their course. 
In a series of horizontal sections, followed from the dorsal towards the ventral 
side, we first come, in the section passing through the middle of the eyes (PI. x1. fig. 9), 
upon two symmetrically disposed groups of ganglion-cells situated behind the eyes, 
and separated from one another by a distinct interval. At a deeper level, but still 
not completely below the eyes, the position taken by the two groups of cells is 
* Semper’s figure and short account of the brain include some of the paired non-nervous cells situated in front of 
and behind it. 
t Loc cit. p, 22. 
