394 
Tetrarhynclms erinaceus 
some underlying tissue; and since the subcuticular cells have a 
glandular appearance and send prolongations towards the cuticle just 
as unicellular cells are situated with regard to the surface on which 
they open, they have had this function ascribed to them. If we study 
the development of the genital ducts we find that these are at first 
apparently epithelial in nature, that is they have walls which are 
composed of two layers of rounded cells lying in close contact with 
each other. By-and-by a homogeneous membrane appears between 
these two layers of cells, and the latter then begin to degenerate, so 
that the wall of the duct finally becomes a membrane of homogeneous 
structure, and with cell nuclei on its outer and inner surfaces. It is 
supposed that the subcuticular cells are related to the cuticle in much 
the same way as the pseudo-epithelial cells are related to the developing 
genital ducts. But there does not appear to be any direct evidence in 
favour of either hypothesis ; and it seems quite as likely that the 
wall of the duct is laid down as a secretion from the surrounding 
parenchyma, and that the cells, or nuclei, are functional in subserving 
the nutrition of the duct wall. If this is the case we can hardly regard 
the subcuticular cells as directly concerned with the deposition of the 
cuticle, for they bear a very similar relation to the latter structure. 
What then is their function ? Pratt (1909) regards them as indifferent 
cells, or embryonic residual tissue, which develops into other specialised 
structures as the Cestode grows in size ; or perhaps as glandular cells 
which secrete some substance acting as a means of defence against the 
digestive juices of the alimentary canal of the host. Now some such 
means of defence must, it appears to me, be postulated in the case of 
a parasitic Trematode or a Cestode but though the view is a probable 
one it cannot be backed up by direct evidence. Against it is the 
appearance of the cells, which is not a glandular one; and it seems 
to me that it is more likely that they are trophic in function and have 
to do with the nutrition of the cuticula. 
The integumentary muscles. These are present as two main series 
and are represented in PI. XXIII, figs. 19 to 21. Fig, 20 depicts part of 
a section made tangentially to the surface of the integument: it passes 
through the latter just beneath the cuticle and is thick enough to 
include both the longitudinal and transverse muscle layers. The 
ti'ansverse fibres are situated externally just beneath the basement 
membrane and are arranged as an uniform layer of very fine fibres. The 
underlying longitudinal fibres are much coarser and they run in irregular 
bundles enclosing interspaces. In the region of the scolex and in the 
