390 G. A. MacUallum, 



York. In the course of these studies it has seemed pretty well ar- 

 ranged by Nature that the parasites of the gills of fish are placed 

 there according to families — for instance the Microcotylidae are 

 seldom or never fouiid on the same fish as the Octocot3'lidae or 

 Acanthocotylidae. At least it is my experience that in such a fa- 

 mily of fish as the Salmonidae the parasites on the gills will almost 

 invariably be found to be Octocotylidae and so on. 



Microcoti/le macrotn 'a. 



In the previous paper mentioned, a description of this form 

 was given which, owing to insufficient material, was uncertain or 

 incomplete in places. Since that time study of new material has 

 made it possible to confirm or complete some of these observations. 

 The description of the mouth, its suckers, and of the peculiar Posi- 

 tion of the small pharynx-like structure on the dorsal surface of the 

 dig-estive canal is all correct, There is a rather twisted or tortuous 

 canal penetrating the pharj-nx but it seems that there is also a 

 freer and unguarded opening from the mouth directly into the 

 digestive tract. 



The intestinal coeca are extraordinarily widely raniified through 

 the body, their diverticula opening on both sides of the main line of 

 the Channel and extending outwardly quite to the skin. Among 

 these diverticula and in very close contact with them lie the other organs. 

 There is hardly any lining to be made out even with high powers. 

 The epithelial cells must be extremely thin and flat for they can 

 nowhere be clearly discerned. 



The parenchyma throughout except in the ventral part of the 

 caudal disc is composed of large polygonal cells with relatively 

 minute nuclei. In the caudal disc the parenchyma is largely re- 

 placed by muscle and what cells are present fill elongated Spaces 

 among these fibres. 



The genitalia are precisely as described before but with the 

 new material it was possible to trace in serial sections the con- 

 tinuity of the vitelline ducts with the two vaginal tubes which join 

 as they approach the vaginal orifice and pass through a thick 

 muscular mass to open on the dorsal surface. 



In two reconstructions of the genitalia from these new serial 

 sections the duct figured at K in fig. D and designated as possi- 

 bly the Canalis genito-intestinalis could be traced out precisely as 



