Haswett—A Monograph of the Temnocephalee. 115 
to be presently deseribed—are in the position indicated above.* The one is, then, 
the nucleus of a cell the body of which forms the wall of the exeretory sac ; it is the 
nucleus of a cell which is fexforated by the excretory sac or terminal part of the 
excretory system of passages. The large size of the sac (about ‘4mm in Zemnocephala 
fasciata in the contracted state) might at first sight seem to stand in the way of 
looking at it as a single cell; but, as will be shown presently, there are other 
cells of comparable size connected with the excretory system. The second nucleus 
is the nucleus of an elongated cell which zs perforated by the main canal: and as the 
wall of the canal and its branches is completely continuous we are justified in 
regarding this as a case like that of the unicellular glands described above, in which 
the cell is drawn out to a very great length. Only a very limited number of nuclei 
oceur in the course of the main canals of 7° fasczata; so that, so far as the main 
trunks are concerned, the latter seem to be perforations in a small number of greatly 
produced cells. Each tentacular vessel in 7. Dendyi (PI. x. figs. 17 and 18) also 
perforates a cell the substance of which is permeated by a plexus of capillaries ; and 
in 7. fasciata there are several nuclei in the wall of each of these tentacular branches. 
The excretory canals in 7ennocephala are, in fact, intracellular as postulated by Lang 
for Trematodes in general ; but in a considerable part at least of their length they 
are perforations in the substance of only a Limited number of greatly produced cells 
(PI. x. fig. 16) united together in the form of a narrow branching cylinder. This 
view of the matter may seem somewhat paradoxical at first sight ; but to my mind 
the facts already described with those yet to be mentioned can bear no other 
interpretation. 
Weber describes and figures the course of the principal canals as they are found 
in 7. Semperi,; and the differences between this and the Australian species are not 
of an important character. The general arrangement is as follows :—The main canal 
into which the sac leads soon bifurcates to form two trunks, from which large vessels 
pass backwards and forwards, giving off numerous branches. The anterior trunks 
are connected across the middle line by a transverse arch, from which are given off 
in front a series of large branches running along the axes of the tentacles. Similar 
transverse arches connect the posterior trunks. In the details of the branching and 
anastomosing of the canals there is a considerable amount of difference between the 
various species. The arrangement observed in 7. Nove-zealandi@ is shown in Plate 
KIetigs A. 
The branches give origin to a system of canalicules or capillaries, superficial in 
position, of small calibre and thin walls, forming a plexus which is best developed 
* It is rather remarkable that Wright and Macallum describe ganglion cells as applied to the excretory vesicles of 
Sphyranura Osleri (l.c. p. 20). It seems to me not improbable that they have fallen into the same error as myself. The 
explanation of iny figure 9, Pl. xxr., with the nuclei external to the sac and the duct, is that what is represented is in 
reality merely the /wmen of the sac and of the duct. 
