THE NEOSPORIDIA 419 



as a floating plasmodium in which sporonts arise, each of which produces 

 sixteen spores ; it has, however, been referred by its discoverer to the Blasto- 

 genea. 



Order IV. : Sarcosporidia. The parasites of this order are con- 

 sidered at present to constitute a single genus, Sarcocystis, with 

 numerous species. In contrast to the three orders of Cnidosporidia 

 dealt with in the foregoing pages, the Sarcosporidia are pre-eminently 

 parasites of the higher vertebrates, more especially of mammals, 

 occurring occasionally, though rarely, in man (see Darling) ; but 

 they are known also to occur in avian and reptilian hosts, though 

 sparingly. On the other hand, no Sarcosporidia are known to be 

 parasitic in invertebrate hosts of any kind. In their hosts the 

 Sarcosporidia are tissue-parasites, occurring principally in the 

 striped muscles, but occasionally in unstriped. In a few cases they 

 are found in connective tissue, but this appears to be a secondary 

 condition in which a parasite living first in the muscle-fibres becomes 

 free from them at a later period. As a general rule the Sarco- 

 sporidia appear to be harmless parasites, which do not make their 

 presence known by any symptoms of disease, and can only be 

 detected by post-mortem examination. Some species, however, are 

 an exception to this rule, and are extremely pathogenic to their 

 host for example, Sarcocystis muris of the mouse. The extent to 

 which the health of the host is impaired appears to be directly pro- 

 portional to the numbers of the parasite in the body, and conse- 

 quently to the power which a given species may possess of 

 multiplying and overrunning the host. In most species the 

 capacity for endogenous multiplication appears to be extremely 

 limited. 



In spite of the fact that Sarcosporidia are very common parasites 

 of domestic animals, and have been found frequently in man, our 

 knowledge of their structure and life-history is in a very backward 

 state. As a rule Sarcosporidia present themselves as opaque, 

 whitish bodies, usually elongated and cylindrical in form, encysted 

 in the muscle-fibres of the infected animal, and known commonly 

 as " Miescher's tubes." They are distinctly visible to the naked 

 eye, and often very large. Sarcocystis tenella of the sheep reaches 

 a length of 16 millimetres, while in the roebuck (Cervus capreolus) 

 cysts of 50 millimetres in length are recorded. The Miescher's 

 tube, when examined microscopically, is seen to be a body of 

 complex structure, and consists chiefly of vast numbers of sickle- 

 shaped spores " Rainey's corpuscles " lying in clumps or bunches 

 contained in chambers separated off from one another by partitions. 

 The whole organism is enclosed by a distinct envelope, often ex- 

 hibiting vertical striations, and the partitions between the chambers 

 containing the spores are continuations of the envelope. The exact 



