THE MYXOSPORIDIA, OR PSOROSPERMS OF FISHES. 253 



thus caused. Moreover they conform very closely to the shape of the 

 cysts. This is especiaTly well shown where a cyst situated in tlie center 

 of a j^roup has shelled out, the surrounding cysts, preserving the shape 

 of the cavity. 



In this species, under influence of cold, concentrated sulphuric acid 

 (which dissolves the tail) the valves separate, the divergence appear- 

 ing always to begin at the posterior end. The appearances seem to 

 favor the view that such divergence was the result of the previous 

 solution of the tail, the 2 lateral pieces of which would thus act as a 

 splint. As, however, examination of uutailed species (in which I sus- 

 pected the lateral pieces might exist witliout the median) failed to show 

 evidence of the existence of the lateral pieces or even of the constancy 

 of the initial posterior divergence, this function of the tail must be 

 regarded as dubious. In any case, at least, one other causal factor must 

 be involved in valve separation, as iodine, which produces separation 

 of the tail, does not produce separation of the valves. I suspected that 

 this might be exosmotic pressure from within, and attempted to produce 

 valve separation by the action of strong glycerin used after iodine 

 had detached the tail, but the results were indecisive. 



This species is particularly interesting as exhibiting decided supero- 

 inferior asymmetry, the superior valve being conspicuously more con- 

 vex, and the supero-median cornu i)rojecting farther forward. It is 

 also important to note that the tail is not a shell process, but is, on the 

 contrary, an independent structure with distinct optical and chemical 

 characters. 



76. Myxoboliis sp. incert. 



Psorospemisof Coreyonusfera, Claparetle, 1874, in Lunel'sHist. nat. despoissous 

 tl. bassin du L^man, p. 114. 



Cyst. — A single one seen, 1 mm. in diameter; contents entirely differ- 

 ent from those of the other branchial cysts, aijproximating to, without 

 being perfectly identical with, those of the cysts of the muscles of the 

 same fish. 



Myxosporidium unknown. 



Spore. — Distinguishable from those of the muscle cysts by their 

 shorter and usually single tail, which, however, in a great number of 

 individuals was bifurcate at the extremity. 



ll((biiat. — Dranchial arches of Coregonus /era. 



77. Myxobolus cf. linearis. PI. 33, tigs. 5-8. 



Cysts of base of dorsal fin of Amciurus melan, (hirlcj', 1893, Bull. U. S. Fish 

 Com. for 1891, xi, p. 417. 



Cyst. — Subs[)herical, about 1 mm. in diameter, 7 in number in a row 

 at the bases of the spines of the second dorsal fin. 

 Myxosporidium unknown. 

 Spore. — Body lanceolate; length of body, 11) /<; breadth, 5 or 6 /<; 



