Haswett—A Monograph of the Temnocephalee. 105 
It does not vary much in thickness in different parts, and is always very thin—a 
single layer in 7. Dendyz. The layer of longitudinal fibres, which varies considerably 
in thickness, but is always much thicker than the circular layer, is divided into a 
number of parallel bundles by slight interspaces of parenchyma usually containing 
pigment. On the ventral side of the body this longitudinal layer is much thicker 
than on the dorsal; on both surfaces it thins out towards the lateral borders ; 
on the ventral side it is divided into irregular narrow bundles, which are more 
numerous laterally ; on the dorsal side it is also divided, here and there, by narrow 
strands of parenchyma. Weber rightly points out that the fibres of the longitudinal 
layer are by no means all exactly longitudinal. The prevailing longitudinal direction 
of the fibres is particularly modified on the ventral side, where the division of the 
layer at the bases of the tentacles and at the sides of the mouth and genital aperture 
produce a greater or less degree of obliquity. 
These muscular fibres of the body-wall are usually angular, sometimes rounded, 
in transverse section, ‘004mm. in diameter in the case of the longitudinal, rather less 
in that of the cireular fibres. They are devoid of nuclei, finely striated in the 
longitudinal direction, embedded in finely fibrillar interstitial matter. In cross- 
section they sometimes appear to contain an axial darker core surrounded by a 
clearer cortical substance. 
The account of the musculature in Zemnocephala brevicornis, given by Brandes 
in the paper already quoted, is in certain important respects different from that 
given above. He speaks of the fibres as hollow, or as giving the impression of a tube 
“durch Anordnung der kontraktilen Substanz im Umkreise der urspriinglichen 
Zelle.” This is particularly distinct, he states, in the longitudinal layer, which, he 
adds, “ Haswell durchaus falsch abbildet und schildert.” ‘Auch hier treten die 
kontraktilen Rohren zu einem auf Querschnitten netzartig erscheimenden Gewebe 
zusammen das sich ganz allmiihlich in das parenchymatische Bindegewebe fortsetzt. 
Mir scheint der letztere Umstand fiir die Genese des Trematodenparenchyms, die 
meines Erachtens durchaus noch nicht aufgeklirt ist, beachtenswerth zu sein ; ich 
werde bei anderer Gelegenheit darauf zuriickkommen.” 
The above account of the longitudinal muscle-layer is totally at variance with 
what I have found in all the species I have studied, and I am disposed to think that 
this may be due to the imperfect state of preservation of the specimens. To this is 
certainly due the description of the muscular fibres as having the appearance of tubes 
—an appearance which I have repeatedly observed in specimens the preservation of 
which had been doubtful, but which is rarely seen in specimens that have been 
properly fixed by means of a corrosive sublimate or osmic acid solution : at most this 
is a preparation phenomenon. 
Lb 
