EntoZoa of UriliKh Murine Fis/ies. 23 



OasUro$tomum tri'yUe, van Beneden. (PI. I. fig. 6.) 



Numerous examples df this 8j)oc'ie3 wore iViquiiitly obtninoJ 

 in the iiitesliu*' of Triijla t/unian/in. 'llicy nieasuroJ 

 l-IJT) mm. in leii;;tli. A tew immature speciuieu.s were iilso 

 present under 1 mui. in lenj^tli. Whether they are really 

 identieal with the Gasterostotniiin trigia: oi van Beneden* is 

 duubttul. 



The shape is somewhat more elontjated than is usual in 

 the genus, so liiat the breadth is only J-j[ of tho length. The 

 wliDle surface of the body, (piito to the posterior enil, is covered 

 with numerous spines. 'i'hese are lar^i^e and strong about 

 the midtlie part of the body, but decrease in size towards each 

 end and particularly towards the hinder end. 



Tiie anterior sucker is subterminal and com|)aratively large, 

 having a diameter of "2'6 mm. in a sj)ecimen 3 mm. long, 

 t. e. j'.j of the boiiy-length. It has a thick rin),but a shallow 

 cavity. The po.>iterior sucker (pharynx) is small and in- 

 signiHcant, nna^uring only '12 mm., i. e. almost exactly half 

 the anterior sucker. It is round and is situated just behind 

 the midtlle point of the body. 



The most characteristic feature of the species is entirely 

 omitted in van Beneden's ligure. This is a peculiar fan- 

 shaped structure surmounting and overhanging the anterior 

 sucker. The upper surface of the structur(3 is thrown into 

 five ridges, with depressions between, all radiating from a 

 point correspond! r)g to the centre of the sucker. These ridges 

 project over the dorsal edge of the structure and appear as 

 snjall symmetrically arranged papillae. The two coiners of 

 the edge overhanging tho sucker also appear to be raised as 

 papillai, but they are not so prominent. The structure is 

 thus seven-pointed, tive of the points being dorsal and two 

 ventral, overhanging the sucker. In none of my specimens, 

 either in the living state or preserved, were the papilliB much 

 extended, and indeed in some of them they were so small 

 ns to be almost invisible. This latter fact may account for 

 van Bmedi-n's failing to observe them. These pupillee are 

 evidently homologous with the much more exaggerated 

 lentacle-like papilhe of Gasterostomum jiinbrialum. 



The alimentary system consists of a short simple sac opening 

 from the posterior sucker and directed forwards. 



The testes are two tairly large rounil or oval bodies, situated, 

 one directly behind the other, on the right side of the body 

 and not far behind the posterior sucker. The cirrus-pouoh is 



• M^m. Ac. Roy. IV'lp. x.xxviii. pi. iii. fig. lo. 



