THE SPOROZOA 277 



therefore, the importance often attributed to it. A large number 

 of forms are tissue parasites, especially the numerous species of 

 Myxobolidae amongst Phaenocystes, and many Cryptocystes. Hence 

 the Myxosporidia, together with the Sarcosporidia, have been termed 

 Histosporidia (Labbe) or Histozoa. These terms cannot, however, 

 be used in any but a physiological sense, as a great many Phaeno- 

 cystes, especially in the families Myxidiidae and Chloromyxidae, occur 

 floating freely in the internal cavities of certain organs. It is 

 therefore more convenient to distinguish at the outset between 

 species living freely, on the one hand, and those infecting cells or 

 tissues, on the other hand. The "free" species occur more 

 particularly in the cavities of biliary or urinary organs in Vertebrate 

 hosts ; that is to say, in the gall-bladder, bile-ducts, urinary bladder, 

 or kidney-tubules. No species are known, however, which, like the 

 Gregarines, occur free in the alimentary canal or in the general 

 body-cavity of their host during the trophic period of the life- 

 history. In the organs which they affect they are found floating 

 freely in the urine or bile, or attached by their pseudopodia to 

 the lining epithelium, but they do not injure the cells themselves, 

 except indirectly, as, for instance, when they may be so numerous 

 in a kidney - tubule as to obstruct the lumen, with pathological 

 consequences to the organ. The species which attack tissues and 

 cells may occur in all parts of the body, infesting usually either 

 connective or muscular tissue. The only classes of tissue exempt 

 from their attacks, so far as is known, are bone and cartilage. They 

 are not known to occur in the testis of any host, and though they 

 frequently attack the connective tissue and stroma of the ovary, 

 they rarely penetrate the ovarian follicles, wjiich happens, however, 

 in the case of the silk worm -moth, with the result of producing 

 hereditary infection. Nervous tissue also is very seldom affected 

 by these parasites, but Glugea lophii, Dofl., attacks the ganglion- cells 

 of Lophius piscatorius. It is amongst the tissue -infecting Myxo- 

 sporidia that the most injurious parasites occur. A given species 

 may either restrict its attacks to one particular organ or tract, or 

 it may ravage impartially almost all parts of the body, and as a 

 rule the destructiveness of a parasite is directly proportional to the 

 extent of its range within the body of the host. In some cases, 

 for instance, in that of Myxobolus pfeifferi of the barbel disease, 

 bacteria have been suspected of aiding the Myxosporidian parasite 

 to produce its fatal results ; but according to Doflein, bacteria do 

 not occur in the tumours produced by the Myxobolus until they 

 have reached the stage of suppuration. 



The tissue-infecting forms fall naturally into two subordinate 

 categories. In the first place, the attacks of the parasite may be 

 concentrated at one spot, in which case a cyst is usually formed round 

 it by the adjacent tissues (Fig. 8 1 ). Within the cyst, the body of the 



