PARASITES. 
421 
111 addition to the destruction due to active effort, the Unionidw 
are sometimes killed mechanically by the agglomeration of Dreissensia 
poli/morpha upon their valves, preventing them opening for respiratory 
and other purposes. 
Parasites are organisms which at any period of their lives are depen- 
dent for food, shelter, or protection, upon some other animal or animals, 
with whose welfare they are, therefore, vitally concerned, and con- 
sequently they do not at any time destroy their host, although some 
forms are known to be the cause of castration in Heliv aspersa and 
other species. 
They are not, as formerly believed, present within or upon the body 
only as a result of the diseased or vitiated condition of the blood or 
tissues of the host, and therefore necessarily indicative of an un- 
healthy condition of the body, nor is there a special class or race of 
parasites, corresponding to that of bii’ds, reptiles, or other groups, as 
all tlie classes of the animal kingdom have given off from their inferior 
ranks some branches in this direction, probably consisting of those 
weaker forms which being unable to successfully compete with the 
newer and stronger races which have successively been evolved have, 
in order to avoid extermination, been compelled to adopt a parasitic 
life, their form becoming modified and degenerating by the atrophy of 
the organs unsuited to their adopted life. From the severity of the 
life competition, parasites have become so numerous that there are 
few or probably no animals which do not entertain a more or less 
numerous party. 
The parasitic organisms dependent upon the mollusca are, with a 
few exceptions, little known, and the cycles of life which many undergo 
have only been thoroughly worked out in one or two species, but it is 
certain that many of our native species harbour one or more of these 
creatures and thus provide them with fooil and shelter, while several 
of our mollusks are knowm to be liable to be infested by eight or ten 
quite distinct species, which either affix themselves upon the exterior 
of the body or live amongst the viscera, and may even be found in 
the pericardial cavity. One species at least, though not in this 
country, is infested by a species of fish which makes use of mollusks 
as a nidus for its ova, the embryos being hatched and developed 
amongst the branchial filaments. The young mollusks of the same 
family are, however, themselves parasitic upon fishes, affixing them- 
selves to and deriving nourishment from their hosts. 
