THE SPOROZOA 167 



below. Here the endogenous generations multiply by schizogony 

 in the blood of a vertebrate host, until sporonts are formed, which 

 must be taken up by a blood-sucking insect, such as a mosquito, 

 in order to develop further. In the invertebrate host the exo- 

 genous generation takes place, and the sporonts give rise, by 

 sporogony after conjugation, to a number of gymnospores or 

 sporozoites, with which the vertebrate host is again inoculated. 



In Sporozoa up to the present three modes of infection have 

 been observed. The first and commonest method may be termed 

 casual infection, where there is no intermediate host, and the 

 infection is acquired by swallowing spores accidentally with the 

 food. In the malarial parasites the infection is effected by the 

 inoculative method, through the agency of an intermediate host. 

 The third method is that of hereditary infection, a rare type, but 

 known in at least one instance, the silkworm-disease produced 

 by the myxosporidian parasite Glugea bombycis (see p. 290), and 

 possibly occurring also in the tick- fever parasites of cattle and 

 other mammals (p. 262). In the first of these two instances the 

 parasites penetrate the ovum and produce spores there, which 

 germinate and infect the next generation of the host. It is 

 possible that to these three modes a fourth should be added, 

 which may be termed the contagious method, seen in the parasites 

 which cause certain human skin-diseases (p. 238), but the sporozoan 

 nature of these bodies is by no means demonstrated with certainty. 



Classification of the Sporozoa. At least five well-established orders of 

 Sporozoa are generally recognised the Gregarinida, Coccidiidea, Haemo- 

 sporidia, Myxosporidia, and Sarcosporidia. In addition, there are three 

 orders which are at present less well known and of very uncertain value 

 the Haplosporidia, Serosporidia, and Exosporidia. The organisms 

 formerly known as Amoebosporidia must now be included in the 

 Gregarinida. 



Many ways of grouping these orders into higher subdivisions or sub- 

 classes of the Sporozoa have been proposed. Labbe set up two sub- 

 classes : (1) Cytosporidia, in which the trophozoite is intracellular, either 

 throughout the trophic period or at least in the earlier stages of growth ; 

 (2) Histosporidia, in which the trophozoite is an intercellular tissue- 

 parasite. The Cytosporidia comprise the Gregarinida, Coccidiidea, and 

 Haemosporidia ; the Histosporidia include the Myxosporidia and Sarco- 

 sporidia. The grouping proposed is a natural one, but the distinctions 

 on which Labbe founded it have not the value which he attributed to 

 them, since the young stages of the Histosporidia are intracellular as 

 often as they are intercellular. Labbe now [4] subdivides the class into 

 Cytosporidia, defined as having "no spore, or a simple spore without 

 polar capsules," and Myxosporidia, having " the spore furnished with polar 

 capsules containing an evaginable filament," while the Sarcosporidia are 

 relegated to the Sporozoa incertae sedis. 



