THE SPOROZOA 275 



variations, correlated with great divergence in habitat and mode 

 of life. They are nevertheless a well-defined and homogeneous 

 order, characterised more especially by the following points of 

 organisation and development : The trophozoite is amoeboid and 

 Rhizopod-like ; spore - formation commences at an early period 

 and proceeds continuously during the growth of the trophozoite ; 

 the spores are produced endogenously, i.e. within the protoplasm 

 of the trophozoite ; and each spore always possesses one or more 

 very distinctive structures, the "polar capsules," which have a 

 strong resemblance to Coelenterate nematocysts. These points 

 taken together are sufficient to distinguish one of the Myxosporidia 

 (including under that term the Microsporidia or Glugeidae) from any 

 other sporozoan type. 



(a) Occurrence, etc. The Myxosporidia, and especially their 

 spores, figure in older' zoological works under the names either of 

 " fish-psorosperms " or of " pebrine-corpuscles." The former name, 

 applied to the sub-order PJuienocystes ( = Myxosporidia sens, strict, 

 auct.}, arose from the fact that the Ichthyopsida are the group of 

 animals most favoured by their attentions ; the latter name, 

 denoting various species of Cryptocystes ( = Microsporidia, Balbiani), 

 was given on account of the well-known association of one species 

 with the destructive silkworm-disease, " la pebrine." 



The Phaenocystes are pre-eminently parasites of Vertebrata, 1 and 

 especially of fishes. They are not known to occur in Amphioxus, 

 in Cyclostomes, or in Ganoid fishes, and a few families of Teleostean 

 fishes, such as the Cycloj)terida and Pkuronectidae, apparently do not 

 harbour any Myxosporidia; but with these few exceptions the 

 greater number of, at any rate, the commoner species of Elasmo- 

 branch and Teleostean fishes are subject to their attacks, and not 

 infrequently one species of fish may be infested by four or five 

 different species of Myxosporidia. They occur commonly also in 

 various Amphibia, especially in Anura. In Reptiles they are less 

 abundant, but Myzidium danileivskyi, Laveran, infests the kidneys of 

 tortoises (Emys lutaria and Cistudo europaea), and an undetermined 

 species has been described from the muscles of lizards and tortoises. 2 

 A "psorosperm" is also reported from the crocodile. 3 But up to 

 the present, no Myxosporidia of any kind are known to occur in 

 warm-blooded Vertebrata, in which the Sarcosporidia seem to take 

 their place. 



The Cryptocystes, on the other hand, are most commonly 



1 Exceptions are : Chloromyxumdiploxys, Thi-lohan ( Cystodiscusdiploxys, Gurley), 

 discovered by Balbiani in the moth Tortrix viridana ; an unidentified species of 

 Myxobolus (?) discovered by Lieberkiihn in the Oligochaete Nais lacustris, and 

 figured by Biitschli, Bronn's "Thierreich," PI. 38, Fig. 23 ; and the organisms described 

 by Stole as Actinomyxidia from aquatic Oligochaeta (vide p. 298 infra). 



2 Danilewsky, 1891, and Pfeffer, 1893 ; see Thelohan [113]. 



3 Solger, 1877 ; see Gurley [102]. 



