30 PRACTICAL PARASITOLOGY 



are distinguished by the shape of the adult spore. They are 

 Myxobolus, with round or oval cnidospores ; and Henneguya, the 

 cnidospores of which are drawn out into a long, tail-like process, at 

 the opposite end to that at which the nematocysts are placed. This 

 process is a prolongation of the shell only, which is bivalved and 

 surrounds the entire cnidospore in a double-contoured envelope. At 

 the meridian, along which the two halves of the shell join, its 

 substance is thickened into a kind of roll, which is clearly seen 

 when the cyst is viewed in profile. Different species are recognized 

 by slight differences in the form and size of the cnidospores. The 

 sporoplasm occupies the hinder half of the inner space, and generally 

 projects in a corner-shaped process between the hinder ends of the 

 two nematocysts. In the interior of the sporoplasm is a vacuole of 

 comparatively large size, which stains a deep red-brown with very 

 dilute iodine, or iodine in iodide of potassium. This vacuole is 

 peculiar to the Myxobolides, and serves to distinguish them from 

 other Myxosporides (fig. 5, 6). The nematocysts may be made to 

 project their threads by one of the methods before mentioned. 



The following experiment is interesting as illustrating the process 

 of infection by natural means. Living cnidospores are enclosed in a 

 morsel of filter paper and the little packet is lowered by means of a 

 thread into the gullet of a living fish of suitable species. A glass 

 pipe should first be introduced and the package lowered through 

 that to avoid injury by the pharyngeal teeth. To prevent the 

 loss of the package, a needle is fastened to the other end of the 

 thread and fixed in the muscles of the back with the point running 

 from tail to head. After the packet has been exposed for twenty- 

 four hours to the action of the secretion of the small intestine, it 

 is withdrawn, the same precautions being employed as before. It 

 will then be found that nearly all the cnidospores have opened, 

 releasing the amoeboid sporoplasm, while all the nematocysts have 

 projected their threads. The means by which the young cnidospores 

 leave the intestine of their host is not known, but it is probably by 

 way of the blood-stream. This would account for the frequency 

 with which the gills become infected, the parasites being caught 

 in the capillary vessels of those organs. 



The nuclear conditions are best studied by fixing the cyst contents 

 upon cover-glasses and staining them with dilute haematoxjdin or 

 iron-haematoxylin. If iron-hsematoxylin is used the specimens should 

 be left for a considerable time in both the mordant and the stain, on 

 account of the extreme impermeability of the shells. But even then 

 it will be found that only a certain number of the cnidospores take the 

 stain, the rest remaining uncoloured. Where the method is suc- 

 cessful, however, it is possible to distinguish the two sporoplasm 



