636 Mr. D. Sharp and Mr. F. Muir on the Comparative 
or concomitantly with the changes as to the invaginated 
genital tube. 
The complete invagination of the male structure in 
the enormous majority of forms is a marked feature of 
Coleopterous anatomy. Another trait of the Order is 
the extraordinary extent to which chitinisation is carried. 
The external parts of Coleoptera are in some cases harder 
than bone, and in these cases the internal phragmas and 
apodemes may share in the hardness, as also the male 
genital tube. For instances we may mention the chitin- 
isation of this structure in the Histeroid genus Oxysternus, 
and the Buprestoid Huchroma. <A further development 
of the genital tube is exhibited by elongation, and by 
chitinisation. We have just mentioned examples of its 
perfect hardness, and as specimens of its elongation may 
mention the long flagella so frequently met with, and 
the remarkable elongation of the sac (or stenazyg0s) 
in Humolpus, where it is about 14 inches long. Turning 
now to the question of the origin of the sclerites of 
the tube, we know from the structure of the body wall 
that exposed large surfaces become very strongly chitinised 
while immediately contiguous parts remain delicate mem- 
brane. The chitinisation takes place by the intermediary 
of hypodermal cells, and it may well be that the reason 
for parts remaining membranous is due to creases prevent- 
ing the proper development of hypodermal cells there, and, 
possibly, their extension in certain directions. 
As the genital tube became elongated it would in the 
invaginated condition be crumpled and creased, and the 
formation of separated sclerites on it may probably have 
been to some extent determined by the nature of these 
foldings. 
We make these suggestions with a view to getting 
the student to realise the probability that the develop- 
ment of the genital tube is due to factors that are on the 
whole similar to those that have determined the structures 
of other parts of the body. The factors are not really 
known. The phenomena of chitinisation are indeed 
specially obscure, and we are not aware that any one has 
offered an explanation of the fact that Histers are hard and 
Malacoderms soft. Neither do we pretend that there is a 
perfect co-relation between the chitinisation of the sclerites 
of the body wall and those of the genital tube: in fact we 
are well aware that in some cases the opposite is true. 
