154 THE SPOROZOA 



classed amongst Siebold's Protozoa, and identified Siebold's vesicle as the 

 cell-nucleus. His views were still further borne out by the important 

 observations of Stein, who in 1848 first demonstrated clearly the relation 

 of the pseudonavicellae to the reproduction of the Gregarines, which he 

 placed as a class Symphyta of the Protozoa. The views of Kolliker and 

 Stein have gradually obtained universal assent, especially after the demon- 

 stration by Lieberkiihn in 1855 of an amoeboid phase in the life-history, 

 and no one now doubts the position of Sporozoa amongst the Protozoa. 

 Nevertheless, for some years this view was energetically combated by 

 various authors, who could not bring themselves to regard the Gregarines 

 as adult, independent organisms. Chief amongst the opponents of the 

 Protozoan theory were Henle, Bruch, and Leydig, who believed that 

 Gregarines were in some way connected with the embryonic stages of 

 Nematodes or threadworms, and more particularly of the genus Filaria. 

 In the course of time, and with increase of knowledge, this theory died a 

 natural death, and it became evident that any associations of Gregarines 

 and Nematodes, or resemblances between them, were of a purely accidental 

 and superficial kind. Looking back, however, upon these controversies, 

 now only of historical interest, it is not a little remarkable that in very 

 recent times a curious nematode-like Sporozoon (Siedleckia nematoides, 

 Caull. and Mesn.) should have been discovered, which, had it been known 

 in the fifties, might have inclined the balance of zoological opinion strongly 

 over to the side of the Nematode theory. 



A retrospect of the history of our knowledge of Sporozoa further 

 brings into prominence the fact that, as an obscure group of no obvious 

 practical importance, they did not for a long time appeal to the considera- 

 tion of the "common-sense" Englishman. Until comparatively recent 

 times, practically the only contributions to sporozoan literature in this 

 country were those of Lankester, who, besides other forms, discovered in 

 1872 the organism, parasitic in the blood of the frog, which at a subse- 

 quent date was named by him Drepanidium ranarum. This discovery, 

 and that of Laveran, who a few years later made known to science the 

 malarial parasites of human blood, laid the foundations of our knowledge 

 of the Haemosporidia, a group of such importance, from the practical 

 point of view, that they have been the cause of focussing the attention of 

 medical men, no less than of zoologists, in all countries upon the Sporozoa. 

 Indeed, so great is the interest which these parasites excite at the present 

 time, on account of their pathogenic properties in man and beast, that 

 now scarcely a month passes without the publication of some discovery 

 relating to them, and the study of the Sporozoa bids fair to assume in 

 the near future a position of importance scarcely secondary to that held 

 by the science of bacteriology. 



The Structure and Life-history of a Typical Sporozoon. As an 

 example of the Sporozoa and of the characteristic features of their 

 life-cycle, we select for detailed description the common Monocystis 

 agilis, Stein l (Fig. 2), a Gregarine parasitic in the sperm-sacs 



1 With regard to the proper name of this species, there is a certain amount 

 of confusion and uncertainty, which is none the less regrettable because of so 



