Ixxii REPORT—1858. 
thread-worms, which had bored their way tirough the soft skin into their 
interior. , 
The long hair-worm of fresh-waters (Gordius aquaticus), vulgarly con- 
ceived to be the result of a metamorphosis of the hair of a horse’s tail, passes 
its early life as a parasite in the body of an insect. 
But many entozoa acquire their full or sexual development, not as free 
worms, but within the body of another animal, and of a species distinct from 
that in which they had passed the early or larval stage of their existence. 
The trematode parasite of a water-fowl produces eggs, from each of which 
is hatched a ciliated infusorial-like young. These young escape into the 
water, and there swim about by their vibratile cilia, like Jnfusoria ; some of 
them enter the body of a water-snail; but they are merely the locomotive 
envelope of a differently-shaped smooth-skinned organism, resembling in its 
simple uniform granular structure a Gregarina; and the function of that 
ciliated envelope was to introduce the Gregarina into the body of the water- 
snail. So introduced, the growth of the gregariniform parasite proceeds, and a 
progeny is seen to arise in its interior, by the development of several of the 
contained germ-cells into embryos: it proves, indeed, to be a mere cyst for 
such, as the infusorial larva had been a cyst for it. The embryos gradually 
acquire the form of a Cercaria. These escape from the cyst, bore their way 
out of the snail, and disperse themselves as free swimming tadpole-like 
animalcules in the water. No sexual organs exist in these ‘ Cercarie,’ any 
more than in their animated ‘coat’ the Gregarina, or in their ciliated 
‘great-coat’ the infusorial embryo. After the larval Cercarte have passed 
some time in the water, first creeping and then swimming about with great 
restlessness, they either enter directly the body of a water-fowl, or bore 
their way into some aquatic insect, or they may fail in both these instinctive 
efforts and remain in the water. In any case they undergo a metamorphosis. 
The Cercaria gathers itself up into a ball and exudes a mucous secretion 
from its surface, which soon hardens; and, since the worm, inside this mucous 
mass, turns round without stopping, it invests itself with a kind of egg-shell : 
during this process the tail is cast off. 
Should this process take place within the body of an insect, the encysted 
Cercaria might be introduced into the body of an insectivorous bird or beast. 
In the act of digestion by the engulpher the body of the insect is destroyed, 
together with the capsule of the cerearian pupa; but this by virtue of its 
vitality remains unharmed, and is thus transplanted into a sphere fitted for its 
further change into a sexual entozoon of the Trematode or ‘ fluke-worm’ order. 
Then again commences the strange and complex genetic cycle from the 
Harveian point—the impregnated ovum. 
Three different species of animal may contribute—two are essential—to 
the successful progress of the ordinary and parthenogenetic processes of pro- 
pagation manifested by the three distinct forms of Infusory, Gregarina and 
Cercaria, intervening between the egg and the perfect parasitic fluke-worm. 
