SPOROZOA, a large and most important section of the Protozoa, all the members of which are exclusively parasitic in habitat. They are of extremely widespread occurrence; there is hardly one of the chief classes of animals which does not furnish hosts for these parasites, scarcely one of the common tissues or organs of the Metazoan body which may not be liable to infection. Sporozoa differ greatly as regards the effects which they produce upon their hosts. In many, perhaps in most, cases the general health of the infected animal seems to be unimpaired, even though the parasites may be fairly abundant. Some, however, give rise to dangerous or fatal diseases, while others may cause ravaging epidemics; instances of these are given under the various orders.
Correlated with the mode of life are the two features characteristic of all Sporozoa: (a) They absorb only fluid nutriment, osmotically, and so lack any organs for ingesting and digesting solid food; and (b) they reproduce by sporulation, i.e. the formation of minute germs, which are in most instances very numerous and are often enclosed in firm protective envelopes or cases, each case with its contents forming a spore. In addition, the great majority have also another method of reproduction, for increasing the number of the parasites in any individual host; this is distinguished as multiplicative or endogenous reproduction, from the propagative or exogenous method (by means of the resistant spores), which serves for the infection of fresh hosts and secures the dissemination and survival of the species. Further, most if not all forms of Sporozoa undergo sexual conjugation at some period or other of the life-cycle.
Beyond this, however, it is impossible to generalize. In response to the exceeding diversity of habitat and of the conditions of life, the parasites exhibit manifold and widely-different types of form, organization and life-history. The recognition of this fact is expressed, at the present day, by the division of the Sporozoa into several well-defined orders, which are grouped in two main divisions, each containing more or less closely related forms. One of these groups consists of the Gregarines, Coccidia and Haemosporidia (qq.v.). The other comprises the Myxosporidia, Actinomyxidia, Sarcosporidia and Haplosporidia, the parasites included in the last named order being of comparatively simple structure, and probably near the base of this section. There are, in addition, various other forms (Sero- and Exo-sporidia), also primitive in character, but which are as yet too insufficiently known for it to be certain whether they are of distinct ordinal rank, or should be placed with the Haplosporidia.
The nomenclature assigned to these two principal divisions of the Sporozoa by different writers has varied according to the particular character on which they have primarily based the arrangement. Of late years, the terms Telosporidia and Neosporidia, proposed by F. Schaudinn (1900), have been most in favour. In the Telosporidia (comprising the Gregarines, Coccidia and Haemosporidia), sporulation does not begin until the close of the vegetative or trophic period, i.e. until growth has ceased; in the Neosporidia (including the remaining orders) growth and sporulation go on coincidently. Recently, however, considerable doubt has been thrown upon the general occurrence of this latter condition in certain Myxosporidia (Microsporidia); and the present writer adopts as preferable, therefore, the terms Ectospora and Endospora (qq.v.), invented by E. Metschnikoff and made use of by F. Mesnil (1899), which indicate a universal distinction between the two groups in their manner of sporulation. This distinction is probably the most fundamental one, and itself supports a conclusion which is, on other grounds, becoming more and more likely, namely, that these two divisions are not related phylogenetically; but have, on the contrary, a radically different origin. In other words, under the heading Sporozoa, as at present used, are included two entirely independent series of Protozoan parasites; the general resemblances which these exhibit are due to convergence brought about by their specialized mode of life.
The most recent and comprehensive account of the group is that by E. A. Minchin (in Lankester’s Treatise on Zoology, pt. i., London, 1903), to which the present writer is much indebted; another useful treatise is that of F. Doflein, Die Protozoen als Parasiten u. Krankheitserreger (G. Fischer, Jena, 1901). Earlier accounts are those of M. Lühe, Ergebnisse der neuren Sporozoenforschung (Jena, 1900); Wasielewski, Sporozoenkunde (Jena, 1896); Y. Delage and E. Herouard in Traité de zoologie concrète, pt. i., Paris, 1896); E. R. Lankester, art. “Protozoa” in Ency. Brit. 9th ed. (1886), and O. Bütschli in Bronn’s Klassen u. Ordnungen des Thierreichs, I. i. (1882). There is a systematic enumeration of the group by A. Labbé in Das Thierreich, 5. (Berlin, 1899); and the classification and phylogeny are considered by E. Mesnil (Soc. Biol., vol. jub. p. 258, Paris, 1899), and by H. Crawley in Amer. Nat. (1905), xxxix. 607. (H. M. Wo.)