308 THE SPOROZOA 



spreads the infection. Three possibilities, at least, suggest themselves in 

 this connection : 



(1) That the intermediate host is a large carnivore of some kind, e.g. 

 the dog, for the parasites of the pig or sheep. 



(2) That, after death of the host, the parasites are taken up by some 

 carrion - feeding animal, which might be either (a) a vertebrate, bird or 

 mammal ; or (6) an invertebrate, such as the blow-fly or the burying beetle. 



(3) That the infection might be taken on by some internal parasite of 

 the vertebrate host, e.g. a flat-worm or nematode. 



The third supposition is extremely unlikely. The second receives, 

 perhaps, some support from the extremely toxic nature of the parasites 

 themselves, which would be a property acting in their interests, by 

 producing the death of the host. In any case there would still remain 

 the question as to how the parasites reinfect the vertebrate host. It 

 appears to be generally young animals that become infected, since the 

 smallest trophozoite that has been described hitherto was found in a lamb 

 eight months old, and it is extremely probable that the infection is by 

 way of the digestive tract. Possibly, therefore, in the intermediate host 

 the parasites form spores more resistent than those formed in the verte- 

 brate host. A remarkable feature of the artificial infections effected by 

 Smith was the long incubation period 40 to 50 days which elapsed 

 between the actual infection by feeding and the appearance of the parasite 

 in the muscles. Evidently there is much still to be made out about these 

 interesting parasites, and the field is one ripe for investigation. 



(c) Classification. Since Blanchard's genera Miescheria and 

 Balbiania denote merely stages in the life-history of the parasites, 

 they have become nomina nuda, and all Sarcosporidia are placed at 

 present in a single genus : 



Sarcocystis, Eay Lankester, 1882, with the characters of the order. A 

 great number of forms have been seen in different animals, many of which 

 are probably distinct species, but only a few have received specific designa- 

 tions : such are S. miescheriana (Kiihn), from the pig ; S. tenella, Raillet, 

 from the sheep ; S. platydactyli, Bertram, from the gecko ; S. muris, 

 Blanchard, from the mouse, etc. 



(d) Affinities. The nearest relationship of the Sarcosporidia is 

 undoubtedly with the Myxosporidia, and with the sub-order Cryp- 

 tocystes (Microsporidia, Glugeidae) in particular. The affinity is 

 manifested in three points more especially (1) the spore-formation 

 proceeds continuously during the whole trophic stage ; (2) they 

 are cell-parasites ; (3) the spores have a single polar capsule. The 

 Sarcosporidia seem to be, in fact, the representatives of the Myxo- 

 sporidia in warm-blooded hosts, and it is not improbable that in 

 the future the two groups will be more closely united in systematic 

 classifications. 



