nd mammals. Following from the casual method of
infection, the epithelium of the gut or of its appendages (e.g. the
liver [Plate I., fig. 1]) is a very common seat of the parasitic
invasion. But in many cases Coccidia are found in other organs, to which
they are doubtless carried by lymphatic or circulatory channels. In
Molluscs, they often occur in the kidneys (fig. 2); in Insects, they are
met with as "coelomic" parasites, the fat-bodies, pericardial cells,
&c., being a favourite habitat; even the testis is not free from their
attentions in one or two instances, though the ovary appears always
immune.

The parasite invariably destroys its host-cell completely. The latter is
at first stimulated to abnormal growth and activity and becomes greatly
hypertrophied, the nucleus also undergoing karyolytic changes (fig. 4).
The fatty materials elaborated by the host-cell are rapidly used up by
the Coccidian, as nourishment; and at length the weakened and
disorganized cell is no longer able to assimilate but dies and is
gradually absorbed by the parasite, becoming reduced to a mere enclosing
skin or envelope. In some cases (ex. _Cyclospora caryolytica_ of the
mole) the parasite is actually intranuclear, the nucleus becoming
greatly swollen and transformed into a huge vacuole containing it.

The effects of a Coccidian infection upon the host as a whole depend
largely upon the extent to which endogenous multiplication of the
parasites takes place. On the one hand, schizogony may be so limited in
extent as not to cause appreciable injury to the host. This seems to be
often the case in forms infecting Molluscs and Arthropods. On the other
hand, where schizogony is rapid and prolonged, the results are often
serious. For, although any one individual only causes the death of a
single host-cell, yet the number of the parasites may be so enormously
increased by this means, that the entire affected epithelium may be
overrun and destroyed. Thus are occasioned grave attacks of coccidiosis,
characterized by severe enteritis and diarrhoea, which may end fatally.
In the case of the Vertebrates, secondary causes, resulting from the
stoppage of the bile ducts, also help to produce death. There is,
however, one factor in the endangered animal's favour. Schizogony cannot
go on indefinitely; it has a limit, dependent upon the supply of
host-cells, and consequently of nutriment, available. As this shows
signs of becoming exhausted, by the rapid mu