52 
Parasitism 
In discussing the general bearing of the whole phenomenon it need 
not be held that the diminution in size of the gonads and the changes 
in character of the sexual organs are directly connected. The first 
may be due to the mere drain of nutriment by the parasite, the second 
must have a more complex cause and is the expression of radical 
alterations in the metabolism of the host. Recent physiological 
experiment has gone far to establish the existence of internal secre¬ 
tions or hormones produced by many if not all organs (conspicuously 
the different parts of the reproductive system in Vertebrates) which 
maintain a most delicate interrelationship with the general metabolism. 
By some such secretion it has often been supposed that the gonads 
regulated and maintained the secondary sexual characters. It fits 
the facts better, here, to suppose, as do Geoffrey Smith and Heape, 
that the regulation of gonads and secondary sexual characters is due 
to a hormone circulating in the blood not regularly produced by the 
gonad. The action of the parasite in the male is to bring about an 
alteration in the constitution of the hormone, in fact there is substituted 
for the male hormone, partially or entirely, the very nearly related 
female hormone, or possibly an intermediate hermaphrodite secretion 1 . 
The production of female characters in a spider-crab not only in the 
absence of an ovary but in the presence of a testis shows the inde¬ 
pendence of secondary character and gonad, and it is to explain this 
that Smith’s suggestion is put forward, while a very similar conception 
was arrived at even earlier by Heape (1901) 2 from consideration of the 
relations of the reproductive system in Vertebrata. 
It is not easy to explain by what sort of mechanism the parasite 
is able to effect such a change in the metabolism of the host. Possibly 
a specific secretion is manufactured by tbe parasite which reacts with 
the sexual hormone, or, as Smith suggests, the latter is of such an 
unstable constitution that the mechanical interference of the parasite 
in the metabolism suffices to bring about its decomposition. The first 
hypothesis of specific secretion is capable of being put to the proof 
by a series of experiments involving administration of extracts of the 
parasite to a number of uninfected crabs and observation of the effects, 
on the lines so successfully established by Starling and Claypon in 
1 For this idea of a reproductive hormone differing in constitution according to sex 
and capable of alteration in the ease of the male, Smith alone is responsible. It is 
significant that the chemical composition of the blood is known to vary widely in the 
vertebrata with difference of sex. 
2 Heape (1901), Quart. Journ. Microsc. Sc., vol. xliv., p. 66. 
