[Illustration: FIG. 5.--SCHIZOGONY OF _ADELEA OVATA_, A. SCHN. (PAR.
_LITHOBIUS FORFICATUS_).
a-c, [venus] generation; d-f, [mars] generation. a, Full-grown [venus]
schizont (_megaschizont_), with a large nucleus (n) containing a
conspicuous karyosome (ky). b, Commencement of schizogony; the nucleus
has divided up to form a number of daughter-nuclei (d.n). The
karyosome of stage a has broken up into a great number of
daughter-karyosomes, each of which forms at first the centre of one of
the star-shaped daughter-nuclei; but in a short time the
daughter-karyosomes become inconspicuous. c, Completion of schizogony;
the [venus] schizont has broken up into a number of _megamerozoites_
([venus] mz) implanted on a small quantity of residual protoplasm
(r.p.). Each [venus] merozoite has a chromatic nucleus (n) without a
karyosome. d, Full-grown [mars] schizont (_microschizont_), with
nucleus (n), karyosome (ky), and a number of characteristic
pigment-granules (p.gr). e, Commencement of schizogony. The nucleus is
dividing up into a number of daughter-nuclei (d.n), each with a
conspicuous karyosome (ky). f, Completion of schizogony. The numerous
micro-merozoites ([mars] mz) have each a nucleus with a conspicuous
karyosome (ky) at one pole, and the protoplasm contains
pigment-granules (p.gr) near the nucleus, on the side farthest from
the karyosome. (From Minchin, after Siedlecki.)]
[Illustration: FIG. 6.--ASSOCIATION AND CONJUGATION IN _ADELEA OVATA_.
a, Young microgametocyte ([mars] gamc.) attached to a megagametocyte
([venus] gamc.). The nucleus of the microgametocyte gives rise to 4
daughter-nuclei (c) which become (d) 4 microgametes ([venus] gam.). e,
One of the microgametes penetrates the megagamete, which forms a
fertilization-spindle composed of male and female chromatin ([mars]
and [venus] chr.). The other 3 microgametes and the residual
protoplasm of the microgametocyte (r.p.) perish. The karyosome of the
megagamete has disappeared, as such. f, Union of the chromatin of both
elements, to produce the zygote-nucleus (n.zyg.). (From Minchin, after
Siedlecki.)]
[Illustration: FIG. 7.--SPORES OF VARIOUS COCCIDIAN GENERA.
a, _Minchinia chitonis_ (E.R.L.), (par. _Chiton_); b, _Diaspora
hydatidea_, Leger (par. _Polydesmus_); c, _Echinospora labbei_, Leger
(par. _Lithobius mutabilis_); d, _Goussia motellae_, Labbe; e,
_Diplospora_ (_Hyaloklossia_), _lieberkuhni_ (Labbe), (par. _Rana
esculenta_); f, _Crystallospora crystalloides_ (Thel.), (par. _Motella
tricirrata_). (From Minchin; b and c after Leger, the others after
Labbe.)]
[Illustration: FIG. 8.--SPOROGONY AND SPORE-GERMINATION IN _BARROUSSIA
ORNATA_, A. SCH., FROM THE GUT OF _NEPA CINERA_.
a, Oocyst with sporoblasts; b, oocyst with ripe spores; c, a spore
highly magnified, showing the single sporozoite bent on itself; d, the
spore has split along its outer coat or epispore, but the sporozoite
is still enclosed in the endospore; e, the sporozoite, freed from the
endospore, is emerging; f, the sporozoite has straightened itself out
and is freed from its envelopes. (From Wasielewski, after A.
Schneider.)]
Comparing the life-cycle of other Coccidia with that just described, a
greater or less degree of modification is frequently met with. In the
process of schizogony two orders of division sometimes occur; the
parent-schizont first divides up into a varying number of rounded
daughter-schizonts (schizontocytes), each of which gives rise, in the
usual manner, to a cluster of merozoites,[3] which thus constitute a
second order of cells. Siedlecki (1902) has found this to be the case in
_Caryotropha mesnilii_ (fig. 4), and Woodcock (1904) has shown that it
is most probably really the same process which Smith and Johnson (1902)
mistook for sporogony when originally describing their Coccidian of the
mouse, _Klossiella_. In _Caryotropha_, a perfectly similar state of
affairs is seen in the formation of microgametes from the
microgametocyte; this is additionally interesting as showing that this
process is neither more nor less than male schizogony.
Coming to the sexual generation, considerable variation is met with as
regards the period in the life-history when sexual differentiation first
makes its appearance. Sexuality may become evident at the very beginning
of schizogony, as, e.g. in _Adelea ovata_ (Siedlecki, 1899), where the
first-formed schizonts (those developed from the sporozoites) are
differentiated into male and female (micro-and mega-schizonts) (see
Plate II., fig. 5). Correspondingly, the merozoites, to which they give
rise, are also different (micro-and mega-merozoites). In one or two
cases sexuality appears even earlier in the cycle, and has thus been
carried still farther back.
The Coccidia, as a whole, have not developed the phenomenon of
association of the sexual individuals prior to gamete-formation which is
so characteristic of Gregarines. Their method of endeavouring to secure
successful sporulation, and thus the survival of the species, has been
rather by the extreme specialization of the sexual process. In place of
many female elements, which the primitive or ancestral forms may be
assumed to have had,[4] there is always, save possibly for one
exception,[5] only a single relatively huge megagamete formed, which
offers a comparatively easy goal for one of the many microgametes.
Nevertheless in the effort to render fertilization absolutely certain, a
few Coccidia have acquired (secondarily) the power of associating; a
state of things which enables those forms, moreover, to effect an
economy in the number of male gametes, only three or four being
developed. Instances are seen in _Adelea mesnili_ (Perez, 1903), _A.
ovata_ (fig. 6), and _Klossia helicina_ (Siedlecki, 1899). It is very
interesting to note that, in the two last cases, unless this association
of the microgametocyte with the megagametocyte occurs, neither can the
former produce male elements (microgametes) nor can the female
individual maturate and become ready for fertilization. (Concerning this
question of association see also GREGARINES.)
In sporogony, great variation is seen with respect to the number of
spores and sporozoites formed; and, as in Gregarines, these characters
are largely used for purposes of classification, under which heading
they are better considered. Usually, the spores (fig. 7) are quite
simple in outline, and not produced into spines or processes;
exceptions are found, however, in a few instances (e.g. _Minchinia
chitonis_). In one case (_Coccidium mitrarium_), the oocyst itself,
instead of being spherical, is curiously shaped like a mitre.
The life-history as a whole is invariably undergone in a single host,
i.e. there is no alternation of true hosts.[6] Schaudinn, in his work on
the _Coccidia_ of _Lithobius_ (1900), showed that the oocysts expelled
with the faeces may be eaten by wood-lice (Oniscus), but when this
happens they pass through the intestine of the wood-louse unaltered, the
latter not being an intermediate host but merely a carrier.
Classification.
The order Coccidiidea is divided into four families, characterized by
the number of sporocysts (if any) found in the oocyst.
Fam. ASPOROCYSTIDAE, Leger. No sporozoites are formed in the oocyst,
the sporozoites being unenclosed (gymnospores).
Genus, _Legerella_, Mesnil. This genus actually conforms to Aime
Schneider's original definition of _Eimeria_, which was founded on
what were really the schizogonous generations of other forms, then
thought to be distinct. In view of the great confusion attending the
use of this name, however, Mesnil (1900) has suggested the new one
here adopted. Two species known, _L. nova_ and _L. testiculi_, both
from different species of _Glomeris_, a Myriapod; the former inhabits
the Malpighian tubules, the latter the testis.
Fam. DISPOROCYSTIDAE, Leger. The oocyst contains 2 spores.
Genus 1. _Cyclospora_, A. Schneider. Spores dizoic, i.e. with two
sporozoites. _C. glomericola_, from the intestinal epithelium of
_Glomeris_, and _C. caryolytica_, from the intestinal epithelium of
the mole, intranuclear.
Genus 2. _Diplospora_, Labbe. Spores tetrazoic. _D. lacazei_, from
many birds, is the best-known species; and others have been described
from different Sauropsida. _D. lieberkuhni_ is an interesting form
occurring in the kidneys of the frog, which it reaches by way of the
circulation.
Genus 3. _Isospora_, Schn. Spores polyzoic. Founded for _I. rara_,
parasitic in the black slug (_Limax cinereo-niger_). Many authors
consider that Schneider was mistaken in attributing many sporozoites
to this form, and would unite with it the genus _Diplospora_.
Fam. TETRASPOROCYSTIDAE, Leger. The oocyst contains 4 spores.
Genus 1. _Coccidium_,[7] Leuckart. The spores are dizoic and the
sporocysts rounded or oval. A very large number of species are known,
mostly from Vertebrate hosts. _C. cuniculi_ (= _C. oviforme_) from the
rabbit (intestine and diverticula), but also occurring sometimes in
other domestic animals; C. falciformis, from the mouse; _C. faurei_
from sheep; and _C. schubergi_, from _Lithobius_ (a centipede), are
among the best-known forms. All of them may cause disastrous epidemics
of coccidiosis.
Genus 2. _Paracoccidium_, Laveran and Mesnil. This genus is
distinguished from _Coccidium_ by the fact that the sporocysts become
dissolved up in the oocyst, thus leaving the 8 sporozoites unenclosed,
recalling the condition in _Legerella_. _P. prevoti_, unique species,
from the frog's intestine.
Genus 3. _Crystallospora_, Labbe. Spores also dizoic, but having the
form of a double pyramid. _C. crystalloides_ from a fish, _Motella
tricirrata_.
Genus 4. _Angeiocystis_, Brasil. Apparently 6 sporozoites, but the
only species, _A. audouiniae_, has only been briefly described; from a
Polychaete (_Audouinia_).
Fam. POLYSPOROCYSTIDAE, Leger. The oocyst contains numerous spores.
There are several genera with monozoic spores, characterized by
variations in the form and structure of the sporocysts, e.g.
_Barroussia_, Schn. (fig. 8), _Echinospora_, Leger, and _Diaspora_,
Leger; most of these forms are from Myriapods.
Genus _Adelea_, Schn. Dizoic spores; sporocysts round or oval, plain.
Several species are included in this well-known genus, among them
being _A. ovata_, _A. mesnili_, _A. dimidiata_; most of them are
parasitic in Insects or Myriapods.
Genus _Minchinia_, Labbe. Dizoic spores; the sporocysts are produced
at each pole into a long filament. _M. chitonis_, from the liver of
_Chiton_ (Mollusca).
Genus _Klossia_, Schn. The spores are tetrazoic (or perhaps polyzoic).
_K. helicina_ from the kidney of various land-snails is the best-known
form. Usually said to have 5 to 6 spores, but Mesnil considers that
the normal number is 4, as is the case in another species, _K. soror_.
Genus _Caryotropha_, Siedlecki. Many spherical spores (about 20) each
with 12 sporozoites. _C. mesnilii_, unique species, from the
spermatogonial (testis) cells of _Polymnia_ (a Polychaete). An
interesting point in the schizogony is the formation of schizontocytes
(see above).
A Coccidian parasitic in the kidneys of the mouse has been described
by Smith and Johnson (1902) and named by them _Klossiella_, on the
ground that it possessed many spores, each with about 20 sporozoites.
Woodcock has shown, however, that the authors were in all probability
dealing with a similar modification of schizogony to that which
obtains in _Caryotropha_. The sporogony of this form (and hence its
systematic position) remains at present, therefore, quite unknown.
There are several doubtful or insufficiently known genera, e.g.
_Bananella_, _Goussia_, _Hyaloklossia_, _Gonobia_, _Pfeifferella_ and
_Rhabdospora_, many of which probably represent only schizogonous
generations of other forms. (For information concerning these see
Labbe, 1897.)
Lastly it remains to mention the extremely interesting forms parasitic
in Cephalopods. For some years these have provided a fruitful source
of discussion to systematists. Here it may be stated simply that their
systematic position and nomenclature were thought to have been finally
settled by the researches of Jacquemet (1903) and Luhe (1902) in the
following terms:--
Genus _Eucoccidium_. Luhe (syn. _Legerina_ Jacq.), Coccidia possessing
polysporous oocysts and lacking schizogony, parasitic in Cephalopods.
Two well-known species: _E. eberthi_ (Labbe), (=_Benedenia_ seu
_Klossia e._ seu _octopiana_), parasitic in _Sepia_, which is tri- or
tetra-zoic; and _E. octopianum_ (Schn.), (syn. _Benedenia_ seu
_Klossia o._) from _Octopus_, which is polyzoic, having 10 to 12
sporozoites. In both forms cysts containing megaspores and
megasporozoites, and others containing microspores and
microsporozoites are found, considered as representing sexual
differentiation thrown back to the very earliest stages of the
life-cycle.
Quite recently much additional light has been thrown upon our
knowledge of these parasites, including a new one, _E. jacquemeti_.
Moroff (1906) has shown that not one but many megagametes are formed,
and fertilized by the microgametes. For this reason he regards them as
Gregarines rather than Coccidia. Further, Leger and Duboscq (1906)
have found that the characteristic coelomic parasites (_Aggregata_) of
Crustacea, generally regarded as gymnosporous Gregarines (i.e.
Gregarines in which the sporozoites are naked) constitute in reality
nothing more or less than a schizogonous generation of these
Cephalopodan parasites, which have thus an alternation of true hosts.
The ripe sporocysts from the Cephalopod are eaten by a particular crab
(e.g. _Portunus_ or _Inachus_, according to the parasite), the
sporozoites are liberated and traverse the mucous membrane of the
intestine, coming to rest in the surrounding lymphatic layer. Here a
large "cyst" is formed, projecting into the body-cavity, the contents
of which give rise to a great number of merozoites. On the crab being
devoured by the right species of Cephalopod, the merozoites doubtless
give rise to the sexual generation again.
As the name _Aggregata_ is much the older, and as, moreover, there is
no longer any reason to retain that of _Eucoccidium_, these parasites
must in future receive the former generic appellation. With regard to
the various specific names, however, they remain quite unsettled until
the life-history is properly worked out in different cases (see also
GREGARINES).
It seems to the writer a much more open question than Moroff and Leger
and Duboscq apparently suppose, whether these parasites are to be
relegated to the Gregarines. For undoubtedly they have many Coccidian
features, and on the other hand they differ in many ways from
Gregarines. The chief feature of agreement with the latter order is
the possession of many female gametes. As already said, there can be
little doubt that this was the condition in the Coccidian ancestor,
and it is by no means impossible that one or two forms existing at the
present day remain primitive in that respect. On the other hand, the
advanced character of the parasitism (the parasites remaining
intracellular up to and including gamete-formation); the entire lack
of the characteristic feature of association; the schizogony, which is
only a very rare occurrence in Gregarines, and which, in the present
case, strongly suggests the process in _Caryotropha_ and _Klossiella_;
and, last but not least, the varying number of the sporozoites (3 in
one form, 10-15 in others), which is very different from the almost
constant number (8) in Gregarines, are all characters in which these
forms agree with Coccidia and not with Gregarines. Having regard to
these points, the writer is inclined, for the present, to consider
_Aggregata_ as an offshoot rather from the Coccidian than from the
Gregarine branch of the Ectosporan tree.
BIBLIOGRAPHY.--The following are some of the important papers dealing
with the order:--G. Bonnet-Eymard, "Sur l'Evolution de l'_Eimeria
nova_, Schneider," _C.R. Soc. Biol._ 52, p. 659, 1900; L. Brasil, "Sur
une Coccidie nouvelle, &c.," _C.R.Ac. Sci._ 139, p. 645, 1904; L.
Cuenot, "_Legerella testiculi_ n. sp., &c.," _Arch. Zool. exp._ (N. et
R.), (3) 10, p. 49, 6 figs., 1902; M. Jacquemet, "Sur la systematique
des Coccidies des Cephalopodes," _Arch. Protistenk._ 2 p., 190, 1903;
A. Labbe, "Recherches zoologiques, cytologiques et biologiques sur les
Coccidies," _Arch. zool. exp._ (3), 4, p. 517, 3 pls., 1897; A.
Laveran, "Sur les modes de reproduction d'_Isospora lacazei_," _C.R.
Soc. Biol_. 50, p. 1139, 1898; A. Laveran and F. Mesnil, "Sur deux
Coccidies intestinales de la _Rana esculenta_," op. cit. 54, p. 857, 9
figs., 1902; A. Laveran and F. Mesnil, "Sur la Coccidie trouvee dans
le rein de la _Rana esculenta_, &c.," _C.R.Ac. Sci._ 135, p. 82, 10
figs., 1902; A. Laveran and F. Mesnil, "Sur quelques Protozoaires
parasites d'une tortue, &c." _t. c._ p. 609, 14 figs., 1902; L. Leger,
"Sur une nouvelle Coccidie a microgametes cilies," op. cit., 127, p.
418, 1898; L. Leger, "Sur la morphologie et le developpement des
microgametes des Coccidies," _Arch. zool. exp._ (N. et R.) (3), 6,
1898; L. Leger, "Essai sur la classification des Coccidies, &c.,"
_Ann. Mus. Nat. Hist._, Marseille (2), Bull. i. p. 71, 4 pls., 1898;
L. Leger "Sur la presence d'une Coccidie coelomique chez Olocrates,
&c.," _Arch. zool. exp_. (N. et R.) (3), 8, p. i., 1900; L. Leger,
"Sur le genre _Eimeria_ et la classification des Coccidies," _C.R.
Soc. Biol._ 52, p. 575, 1900; L. Leger and O. Duboscq, "Recherches sur
les Myriapodes de Corse et leurs parasites," _Arch. zool. exp._ (4),
1, p. 307, 24 figs., 1903; L. Leger and O. Duboscq, "Sur l'evolution
des Gregarines gymnosporees des Crustaces," _C.R.Ac. Sci._ 142, p.
1225, 1906; L. Leger and O. Duboscq, "L'Evolution d'une _Aggregata_ de
la seiche chez le _Portunus depurator_," _C.R. Soc. Biol_. 6o, p.
1001, 1906; M. Luhe, "Uber Geltung und Bedeutung der Gattungsnamen
_Eimeria_ und _Coccidium_," C.B. Bakter (1) 31 Orig, p. 771, 1902;
C.B. Bakter, "Die Coccidien-Literatur der letzten vier Jahre," _Zool.
Centrlbl._ 10, 45 pp., 1903; F. Mesnil, "Sur la conservation du nom
generique _Eimeria_, &c.," _C.R. Soc. Biol._ 52, p. 603, 1900; F.
Mesnil, "Les Travaux recents sur les Coccidies," _Bull. Inst.
Pasteur_, i. pp. 473, 505, 1903; R. Metzner, "Untersuchungen an
Coccidium cuniculi," _Arch. Protistenk._ 2, p. 13, pl. ii. 1903; G.
Moussu and G. Marotel, "La Coccidiose du mouton et son parasite,"
_Arch. Parasitol._ 6, p. 82, 10 figs., 1902; T. Moroff, "Sur
l'evolution des pretendues Coccidies des Cephalopodes," _C.R.Ac. Sci._
142, p. 652, 1906; C. Perez, "Le Cycle evolutif de l'Adelea mesnili,
&c.," _Arch. Protistenk._ 2, p. 1, pl. 1, 1903; F. Schaudinn,
"Untersuchungen uber den Generationswechsel bei Coccidien," _Zool.
Jahrbucher_ (Anat.) 13, p. 197, 4 pls., 1900; F. Schaudinn, "Studien
uber krankheitserregende Protozoen--I. _Cyclospora caryolytica_, &c.,"
_Arb. kais. Gesundh.-amte_, 18, p. 378, 2 pls., 1902; M. Siedlecki,
"Reproduction sexuee ... chez ... _Coccidium proprium_," _C.R. Soc.
Biol._ 50, p. 664, figs., 1898; M. Siedlecki, "Etude cytologique ...
de la Coccidie de la seiche, &c.," _Ann. Inst. Pasteur_, 12, p. 799, 3
pls., 1898; M. Siedlecki, "Etude cytologique ... de Adelea ovata," op.
cit. 13, p. 169, 3 pls., 1899; M. Siedlecki, "Cycle evolutif de la
_Caryotropha mesnilii_, &c.," _Bull. Ac. Cracovie_, p. 561, 5 figs.,
1902; T. Smith and H. P. Johnson, "On a Coccidian (_Klossiella muris_,
gen. et spec. nov.), &c.," _J. exp. Med_. 6, p. 303, 3 pls., 1902;
H.M. Woodcock, "Notes on Sporozoa, I. On _Klossiella muris_, &c.,"
_Q.J. micr. Sci._ 48, p. 153, 2 figs., 1904. (H. M. Wo.)