THE SPOROZOA 151 



general, though by no means universal, rule, each species of 

 Sporozoon is parasitic on a particular species of host, or on a limited 

 number of allied species, and is usually confined to definite organs 

 or tissues of the host. In other words, the 

 various species of Sporozoa, like most internal 

 parasites, have acquired each an organisation in 

 harmony with certain special conditions of life, 

 and, except for a brief period of their developmental 

 cycle, they cannot exist apart from the very definite 

 and limited environment to which they are exclusively 

 adapted. 



The Sporozoa also differ widely as regards the 

 effects they produce upon the animals which harbour 

 them. In many, perhaps in most, cases the general 

 health and vital activity of the host seems to be 

 quite unaffected, even when it contains great numbers 

 of the parasites. But in other cases Sporozoa pro- 

 duce dangerous or even fatal diseases, and may be the 

 cause of ravaging epidemics. Instances of this will be 

 found below, especially under the heading of the Myxo- 

 sporidia. It is sufficient to mention here the various 

 forms of malarial fever in man, now known to be caused 

 by sporozoan parasites of the order Haemosporidia. A 

 still more deadly human disease, namely cancer, has 

 also been referred to the agency of Sporozoa, but this 

 charge has not yet been brought home to them satis- 

 factorily. 



Different species of Sporozoa vary between wide 

 limits as regards size, as well as in other characters. 

 From minute organisms, several of which can be con- 

 tained in a single blood-corpuscle, we find all grada- 

 tions of size up to creatures whose dimensions must 

 be regarded as very considerable, or even gigantic, 

 Fio l in view of the fact that the sporozoan individual 



(sporont) i g > like other Protozoa, a single nucleated cell. 

 v Spyiofit Many of the Gregarines are quite visible to the 

 cieus. (After van Bene- naked eve. and PorospoTd oiaantea (v. Ben.) from 



den, from Lankester.) "h ' 1 Vi f 1 fi t 



thirds of an inch (Fig. 1). In spite, however, of the extremest 

 diversity in size, appearance, organisation, and life -history, the 

 Sporozoa as a group possess certain very characteristic features in 

 common peculiarities which are clearly in direct relation with 

 their habit of life as internal parasites. 



In the first place, their nutriment is always of a fluid nature, 

 consisting of the juices of the host absorbed osmotically at the 

 surface of the body of the parasite, and none of the special organs 



