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 digestive juices, which are thus enabled to reach the spores and cause the rupture of the sporocysts. As the result of instructive experiments, Metzner has shown that it is the pancreatic and not the gastric juice by which this liberation of the germs is effected. The liberated sporozoites creep out and proceed to infect the epithelial cells. The sporozoites (XV.) are from 15–20 long by 4–6  wide; they are fairly similar to merozoites in form, structure and behaviour, the chief point of distinction being that they have no karyosome in the nucleus (cf. above).

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, 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, there is always, save possibly for one exception, 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 .)

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. 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.

The order Coccidiidea is divided into four families, characterized by the number of sporocysts (if any) found in the oocyst.

Fam. , Léger. No sporozoites are formed in the oocyst, the sporozoites being unenclosed (gymnospores).

Genus, Légerella, Mesnil. This genus actually conforms to Aimé 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. , Léger. 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, Labbé. Spores tetrazoic. D. lacazei, from many birds, is the best-known species; and others have been described from different Sauropsida. D. lieberkühni 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. , Léger. The oocyst contains 4 spores.

Genus 1. Coccidium, 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 Légerella. P. prevoti, unique species, from the frog’s intestine.

Genus 3. Crystallospora, Labbé. 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. , Léger. 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, Léger, and Diaspora, Léger; 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, Labbé. 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)