214 BULLETIN OF THE BUREAU OF FISHERIES. 



found in a single host; in others, all the trophozoites in the same fish are at about the 

 same stage of development. 



As has been previously noted by Auerbach (1910) and others, the susceptibility of 

 fish to infection by Myxosporidia varies markedly mth age. Auerbach found that in 

 Gadus virens, which is parasitized by Myxidium bergense, the very yoimg fish were prac- 

 tically free from parasites, while the half-grown fish were much more commonly infected 

 than the adults. The author's experience has been that in general young fishes are rarely 

 infected, while the maximum infection is to be found in nearly grown or adult fishes. This 

 is strikingly shown in the sharks, the gall bladders of which are always infected with one 

 or more species of Myxosporidia, except in the case of very young individuals. In young 

 sharks less than 15 inches in length the author has very rarely found any trace of these 

 parasites. It is also noteworthy that the young fish of any species when infected usually 

 contain only a comparatively few trophozoites, which for the most part show no traces of 

 spores. 



Taking all the above facts into consideration, it is remarkable that such a large 

 proportion of the fish examined should have been found to be infected. 



As regards the site of the parasites within the host, the "free" forms appear to 

 be confined for the most part to the gall and urinary bladders. A few species are found 

 in the urinary tubules and ducts, but so far as known they are never found in the lumen 

 of the digestive tract. 



The tissue parasites are found chiefly in the connective and muscular tissues, 

 although other classes of tissues are by no means immune, and a few species are known 

 to attack the nervous system. The parasite may be concentrated at one spot in the 

 tissues to form a distinct cyst, which is usually visible to the naked eye, or may spread 

 in an irregular manner through the tissue, producing a condition known as "diffuse 

 infiltration." The tissue parasites, although abundant in fresh-water fishes, are rare 

 in marine fishes. They were only met with in a few species, and in every case these 

 were fishes which also occur in brackish or fresh water. 



It is often stated that parasites of the kidneys and urinary bladder are also confined 

 chiefly to the fresh-water fishes, while parasites of the gall bladder are just as character- 

 istic of marine species. With this the author is unable to agree. In marine fishes 

 Myxosporidia were found to be fully as abundant in the urinary as in the gall bladder. 

 On the other hand, such limited observations as were made on fresh-water fishes indicate 

 that they are by no means rare in the gall bladders of these fishes. 



As a general rule, each species of Myxosporidia is confined to one, or at most a few, 

 cfosely related species of fishes, but to this rule there are numerous exceptions, and in 

 some cases the parasites may occur in very different hosts, as, for example, Sphccromyxa 

 balhianii, which occurs in the gall bladder of Motella maculata and Siphostoma floridcB. 



Some genera appear to show a preference for certain organs of the host. The 

 Geratomyxa are chiefly parasites of the gall bladder, although a few species are known 

 from the urinary bladder. In the case of Sphceromyxa all the known species are from 

 the gall bladder. On the other hand, the species of Sphcerospora are, with one excep- 

 tion, all found in the urinary bladder. There is no known instance of the same species 

 inhabiting both the gall and urinary bladders, but the tissue parasites do not seem to 

 be so particular, many species being able to develop in quite different tissues. 



