400 



THE PROTOZOA 



other classes of tissue. A few species are known to attack the 

 nervous system for instance, Lentospora (Myxobolus) cerebralis , 

 cause of " Drehkrankheit " in Salmonidce (Plehn), and Myxobolus 

 neurobius of trout (Schuberg and Schroder). In the tissue attacked 

 the parasite may be concentrated at one spot, so as to form a dis- 

 tinct cyst visible to the naked eye ; or parasite and tissue may be 

 mixed up together in a state of " diffuse infiltration " such that 

 microscopic examination is required to detect the parasite, and as 

 its body becomes used up, to form spores, the tissue becomes in- 

 filtrated with vast numbers of spores lying singly or in groups 

 between the cells. 



In many species of Myxosporidia, on the other hand, the spore- 

 forming plasmodial phase is found in 

 cavities of the body not in any 

 known instance in the lumen of the 

 digestive tract, but frequently in the 

 gall-bladder or urinary bladder of the 

 host. In such cases the parasite 

 may lie quite freely in the cavity it 

 inhabits, or may be attached by 

 its pseudopodia to the lining epi- 

 thelium ; in the latter case the 

 attachment is purely mechanical, 

 and does not involve injury to the 

 epithelial cells. 



As might be expected, the Myxo- 

 sporidia parasitic in tissues are 

 often very deadly to their hosts, 

 and are sometimes the cause of 

 severe epidemics among fishes. 

 Those species, on the other hand, 

 which inhabit cavities with natural 

 means of exit from the body appear to be as harmless to their 

 hosts as are the majority of parasitic Protozoa in nature. 



The adult trophic phase is usually a large amoeba-like organism 

 with a distinct ectoplasm and endoplasm. In some species the 

 ectoplasm, which appears to be purely protective in function, ex- 

 hibits vertical striations, or is covered by a fur of short, bristle-like 

 processes, the nature and significance of which are uncertain as, 

 for example, Myxidium lieberkuhni, the common parasite of the 

 urinary bladder of the pike (Esox lucius). The form of the body 

 changes constantly, with extrusion of pseudopodia, which are 

 used for locomotion to a limited extent, more often for fixation, 

 but never for food-capture. They may, however, by increasing 

 the body-surface, increase also the power of absorption of food- 



FIG. 164. Chloromyxum leydigi, 

 parasite of the gall-bladder of the 

 dogfish, skate, etc. ; trophozoite 

 (plasmodium) in an active state. 

 ect., Ectoplasm ; end., endoplasm ; 

 y., yellow globules in the endo- 

 plasm ; sp., spores, each with four 

 polar capsules. After Thelohan, 

 from Minchin, magnified 525. 



