1911 Encyclopædia Britannica/Ectospora

ECTOSPORA, a homogeneous and natural division of Protozoan parasites included under the Sporozoa; they comprise the three orders, Gregarines, Coccidia and Haemosporidia. The defining character of the Ectospora is that the spore-mother-cells (sporoblasts) are formed at the periphery of the parent-individual (sporont); we may, however, go further, and say that the formation of all the different reproductive elements is uniformly peripheral or exogenous. Two other very general features are (a) that the individual trophozoite is uninuclear, and (b) that growth and trophic activity are finished before the multiplicative or reproductive phase sets in.

There is now little doubt that the Ectospora possess a flagellate ancestry. The principal facts in favour of this view are as follows: the actual ontogenetic connexion known to exist between certain Haemoflagellates and certain Haemosporidia (see Trypanosomes); the possession by many Coccidia of biflagellar microgametes (male elements), whose general structure greatly resembles that of a Heteromastigine Flagellate; the possession by various parasitic Flagellates (e.g. Herpetomonas) of an attached, resting phase, when the parasites become gregariniform, which strongly suggests the attached phase of many young, growing Gregarines; the typical gregarinoid and euglenoid movements of Gregarines and of the germs or other stages of Coccidia and Haemosporidia, which are quite comparable with the contractile and metabolic movements of Flagellates; and, lastly, the exogenous type of reproduction, which is easily derivable from the multiple division of certain Haemoflagellates, and this, in turn, from the typical binary longitudinal fission of a Flagellate.