THE MYXOSPORIDIA OP THE BEAUFORT REGION. 205 



GENERAL ACCOUNT OF THE MYXOSPORIDIA. 



INTRODUCTORY. 



The Myxosporidia are chiefly parasites of fishes, although several have been found 

 in Amphibia and reptiles and possibly in a few invertebrates." So far as known they 

 are all endoparasites, and no part of the fish, except possibly the digestive cavity, 

 appears to be free from infestation. They are often described as being primarily tissue 

 parasites, but this is a mistaken view, due to the fact that the tissue parasites often 

 form conspicuous lesions and may be very injurious to the host. On this account 

 they have attracted more attention, while the less injurious and less conspicuous forms 

 occurring in various cavities in the body have largely been overlooked, although in 

 reality much more common. 



Not only are the Myxosporidia to be looked upon as occurring chiefly in various 

 cavities of the host — at least for the greater part of their existence — but, as has been 

 pointed out by Auerbach (1910), the "free" fonns are more primitive and nearer the 

 original ancestors of the group than the typical tissue parasites. The latter are highly 

 specialized forms which have in all probability descended from species which lived 

 free in cavities of the host. A number of recent writers have urged that the Sporozoa 

 are a polyphyletic group — one division having been derived from the Mastigophora, 

 the other from the Sarcodina. The sarcodine ancestry of the Myxosporidia is strongly 

 indicated by the "free" forms, the trophozoites moving by means of well-developed 

 pseudopodia and often, especially in the younger stages, appearing strikingly like Amebae. 



STRUCTURE AND ACTIVITIES OP THE TROPHOZOITES. 



From what has been said it will be evident that the typical structure of the tropho- 

 zoites is best shown by those forms which spend the greater part of their existence in 

 cavities of the host. In these cases the trophozoites form plasmodia, which either 

 float free in the fluids filling the cavities or are attached to the epithelial lining. Although 

 the Plasmodia of the same species often show considerable variations in form, it is never- 

 theless true that each species is characterized more or less definitely by peculiarities 

 of form and structure, so that in many cases they can be- recognized even in the absence 

 of the spores. This is especially true when applied to the different genera, although 

 even in this case there are numerous exceptions. For instance, in most species of 

 Ceratomyxa the trophozoites are pyriform with a definite anterior end — in Sphceromyxa 

 flat and leaflike, and in Myxidium Amebalike, with short, lobose pseudopodia. In 

 practically all cases the trophozoites when irritated or subjected to unfavorable condi- 

 tions tend to contract into a spherical mass. 



Usually the trophozoites show a well-defined ectoplasm and endoplasm, but in 

 some cases there is no distinct peripheral layer. When distinguishable, the ectoplasm 

 may form a distinct layer all over the body or it may be recognizable only in certain 

 regions. It is denser than the endoplasm, usually clear and homegeneous, and occa- 

 sionally, as in Myxidium lieberkuhni, Sinuolinea dimorpha, or Chloromyxum Isydigi, 

 may be covered with a layer of short hair or bristlelike processes. The more or less 



o The accounts of the occurrence of true Myxosporidia in the invertebrates are very few and fragmentary and badly in need 

 of confirmation. 



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