THE SPOROZOA 299 



forms exhibiting adaptive degeneration due to their parasitic life. The 

 tendency to such degeneration is to be regarded as having reached its 

 culminating point in the tissue-infecting Myxobolidae and Cryptocystes, and, 

 as in other parasites, it goes hand in hand with a great increase of the 

 reproductive power, shown in the large number of spores produced by 

 each individual. 



It is hardly possible in the present state of knowledge to put forward 

 any criticisms upon these interesting speculations until more is known of 

 the relationship of the Myxosporidia to other Sporozoa. At the present 

 they stand very much apart and isolated, and until intermediate forms 

 are discovered linking them to other groups, it is not possible to decide 

 which forms are to be considered as the most primitive in their organisa- 

 tion. As regards the reproductive capacity, however, it may be pointed 

 out that on the whole it is distinctly related to the habitat of the parasite. 

 Thus the Disporea all infest regions of the body in which their spores 

 can pass to the exterior without difficulty by natural channels, and there 

 is no danger of the spores becoming stale or dying off from too long 

 retention within the body of the host, as so frequently happens in the 

 tissue -infecting forms. In the latter, therefore, a much greater repro- 

 ductive capacity is necessary to guard against the danger of staleness than 

 in the former, in which no such risk exists. Not only is it necessary for 

 the tissue-infecting forms to produce more spores, but also to keep up a 

 constant supply of them, so that there may always be ripe spores ready 

 to carry on the race when the opportunity for their dissemination arrives. 

 These considerations do not, however, in any way invalidate Thelohan's 

 conclusions. 



ORDER 5. Sarcosporidia. 



The Sarcosporidia are the least known of all the chief orders of 

 Sporozoa, in spite of the fact that, although restricted in occurrence, 

 and also, apparently, as regards variety of genera and species, they 

 are exceedingly abundant as individuals and very easily obtained 

 as material for investigation. So far as it is possible in the present 

 state of knowledge to characterise the group, their distinctive 

 features are as follows. With the rarest exceptions, they are 

 parasites of the striped muscles of warm-blooded vertebrates, birds 

 and mammals. The trophozoite is an elongated body, which, in 

 the earliest stages hitherto observed, is motionless, and is limited 

 at first by a delicate cuticle, later by a thick envelope of compli- 

 cated structure. Spore-formation commences at an early stage 

 and proceeds during the growth of the trophozoite (hence Neo- 

 sporidia), which may attain to a great size. The spores, which 

 are produced in great numbers, are minute sickle -shaped or 

 spindle-shaped bodies, with a very delicate envelope containing (1) 

 the sporoplasm, with a single nucleus ; (2) an oval striated body, 

 placed at the pole of the spore, and representing, apparently, a 

 polar capsule such as is found in the Myxosporidia. In some 



