). See Herzog-Hauck, _Realencyklopadie_.

COCCIDIA, an important order of Sporozoa Ectospora, parasites possessing
certain very distinctive characters. With one or two possible
exceptions, they are invariably intracellular during the entire trophic
life of the individual. They always attack tissue-cells, usually of an
epithelium, and never blood-corpuscles. Correlated with the advanced
degree of parasitism, there is a complete absence of specialization or
differentiation of the cell-body, and the trophozoite is quite incapable
of any kind of movement. In all cases, so far as known, the life-cycle
is digenetic, an asexual generation (produced by schizogony) alternating
with a sexual one (gametogony). After conjugation of two
highly-differentiated gametes has taken place, a resistant oocyst is
formed, which provides for the dispersal of the species; inside this
sporogony (spore- and sporozoite-formation) goes on.

History.

Hake (1839) was, perhaps, the first to describe a Coccidian, but he
regarded the parasites as pathological cell-products. In 1845 N.
Lieberkuhn pointed out the resemblances to Gregarines, with which
organisms he considered Coccidia to be allied. A year later, H. Kloss
proved the existence of similar parasites in the snail, and attempted to
construct their life-history; this form was subsequently named _Klossia
helicina_ by A. Schneider. The asexual part of the life-cycle was first
described by Th. Eimer in 1870, for a Coccidian infesting the mouse,
which was afterwards elevated by Schneider into a distinct genus
_Eimeria_. The generic name _Coccidium_ was introduced by R. Leuckart in
1879, for the parasite of the rabbit. It was many years, however, before
the double character of the life-cycle was realized, and the ideas of L.
and R. Pfeiffer, who first suggested the possibility of an alternation
of generations, for a long time found no favour. In the first decade of
the 20th century great progress was accomplished, thanks largely to the
researches of F. Schaudinn and M. Siedlecki, who first demonstrated the
occurrence of sexual conjugation in the group; and the Coccidian
life-history is now one of the best known among Sporozoa.

Habitat: effects on host.

Coccidia appear to be confined[1] to four great phyla, Vertebrates,
Molluscs, Arthropods and Annelids; the first named group furnishes by
far the most hosts, the parasites being frequently met with in domestic
animals, both birds a