202 THE ROYAL SOCIETY OF CANADA 



of which are drawn to the same scale. The largest specimen removed 

 from its cyst shows only the beginnings of the reproductive organs 

 and the intestine reduced to about half the size seen in the adult. 

 From these facts it is evident that, whatever may be the fate of 

 the miracidium up to the earliest stage described above, the species 

 reaches its final hosts through smaller fish which are taken as food. 



28. Distomum sp. larv. 



Encysted in the muscles of Perca flavescens. 



Eleven pigmented cysts removed from the flesh of a young Perch 

 were found to contain larval distomes, showing an anterior sucker, 

 prepharynx, small pharynx, short oesophagus and forked intestine. 

 A ventral sucker could not be seen. The probable anlagen of the 

 testes appeared as two clear areas spearated by a transverse bar of 

 opaque-white material and situated in the posterior half of the body. 

 The body was covered, over the anterior end at least, with minute 

 spines running in two diagonal directions. The cysts, themselves 

 (Fig. 24), are ellipsoidal in shape but somewhat flattened in the direc- 

 tions of two circular openings, one on each side, which lead into 

 the interior. The diameter of this cavity is smaller than the outer on 

 account of the very thick walls, composed of a hard transparent 

 substance covered over with a thin transparent membrane in which 

 the black pigment flecks are located. 



29. Cer caria sp. ? 



Encysted in the skin of a Minnow, species not determined. 



Numerous black cysts resembling those just described (Fig. 25) 

 were found to contain cercariae provided with appendages. A well 

 developed oral sucker and pharynx, but no traces of oesophagus nor 

 intestinal coeca were to be seen. The ventral sucker (Fig. 26B) was 

 best seen from a lateral view. In Fig. 26A it is much enlarged, prob- 

 ably owing to injury in the removal of the worm from the cyst 

 followed by the imbibition of water by the very delicate tissues. 

 The short conical tip of the appendage was seen to be retractile. 

 Owing to the extreme difficulty in removing these worms without 

 injury from the hard and very small cysts, nothing further on their 

 anatomy was recorded, excepting that a system of opaque- white and 

 granular lines (represented dark in the figure), much resembling those 

 seen in Diplostomum cuticola, probably constitutes the excretory 

 system. 



I have also noticed that practically all of the small species of 

 fishes together with the young and not a few adults of the larger 

 species are more or less infected with this kind of cyst, which state- 



