REPORT UPON A PARASITIC PROTOZOAN. 181 



found "encysted" individuals on young Leuciscus rutilus, but does not say whether 

 these were in process of segmentation. 



According to this author the injury to the fish is due to the fact that the epithe- 

 lium is injured by the ciliate, and thus places are exposed where fungi (especially 

 Saprolegnia ferax) gain a hold. The latter then retard the function of the skin to 

 such an extent that the fish soou succumb. 



OBSERVATIONS AT THE WORLD'S FAIR. 



The parasite (fig.l) found at the World's Fair is unquestionably an Ichthyophthirius; 

 whether Ichthyophthirius should, however, be given generic rank, as Fouquet and 

 Zacharias have concluded, or whether it should be considered as a subgenus of the 

 genus Holophrya, as Biitschli contends, is perhaps still an open question. Personally 

 I lean decidedly to the view of Fouquet and Zacharias, that Ichthyophthirius is enti- 

 tled to generic rank, but prefer at present nevertheless to follow rather than oppose 

 an authority like Biitschli, unless supported by stronger evidence against his view 

 than I have at present. Accordingly, I have accepted Ichthyophthirius as subgenus. 

 The question of the specific relations between the form I have studied and the animals 

 studied by Fouquet and others will be discussed further on. 



In many respects the observations made agree perfectly with observations of other 

 authors, as given previously; some of my observations may, however, be interesting 

 and worthy of record, as they tend to complete the descriptions reviewed above as 

 well as to explain some differences of opinion expressed by various authors. 



The size of the animals now under consideration agrees approximately with the 

 figures given by Fouquet and others, i. e., up to 0-7 mm or 0-8 mm in diameter. These 

 figures are, however, estimated, as no means for exact measurement were at my dis- 

 posal. The body is round to elongate oval in shape when at rest, but changes its 

 form almost as much as an arnceba. This change is naturally not due to pseudopodia, 

 but to its twisting, turning, and contracting. Deep grooves may extend across its 

 outer surface, or the body may be perfectly smooth superficially; it maybe flat on 

 one surface and convex on the other, or it may be nearly spherical; in fact, it may 

 assume almost any form when in motion. When at rest the body becomes globular 

 or oval, and this should perhaps be looked upon as the normal form, as it agrees with 

 the form of the young. Even when not progressing in the water, but remaining 

 apparently in one place, the animal is in constant motion, revolving not only to the 

 right and left, but in every other direction, that is, dorso-ventrally, ventro-dorsally, 

 diagonally, etc. This may perhaps explain the statements of different authors in 

 regard to the position of the mouth, for, as we saw above, Hilgendorf and Paulicki 

 state that the mouth is absent (in other words, they observed none); Fouquet found 

 the mouth (sucker) terminal and anterior; Kerbert placed it on the left side, and 

 Zacharias on the ventral surface in the anterior third. 



According to my observations, when the animal has assumed an elongated form, 

 one end being more pointed than the other, the mouth (fig. 1) is frequently situated 

 terminally at the pointed extremity, while the nucleus is nearer the other extremity. 

 If we observe the animal for some time, however, we notice that any given point of 

 the surface of the animal may come to lie at the pointed portion; that is, any portion 



