19I3-J '^- Southwell : Parasites from Fish. 97 



his specimens, however, in a Dolphin, which had fish in its stomach, 

 seems to indicate that his form had been derived from fish, and 

 that the worm had either not had time to develop in the Dolphin, 

 or that it did not do so in that host. The condition described by 

 Linton would find a parallel here in the stomach of any predaceous 

 animal (such as Wallago attu) which might eat the eggs, or the 

 mature female of Nandus marmoratus. The only other form, 

 which, as far as I have been able- to ascertain, resembles our species 

 is a larva of D. varicgatum figured by Dr. A. Looss (2). The differ- 

 ences between the two are that our specimens are twice as large, 

 and that the ventral sucker is more anterior in ours than in that 

 figured b}^ Looss. Looss's examples were from the lung of a frog. 

 The life-histor}' of the worms constituting the genus to which our 

 young specimens belong is extraordinarily complicated. Two ex- 

 amples briefly outlined will serve to indicate this fact and wall also 

 illustrate how difficult it is to determine 3^oung forms. 



I. There occurs in Europe a Trematode belonging to the 

 genus Gasierostomum. The eggs liberated from this worm have 

 their first larval stages in certain mussels {Anodonta). In this host 

 the larvae develop into sporocysts, and redia, and cercariaare even- 

 tually formed. If infected mussels are eaten by Belone vulgaris 

 (a Gar fish) the cercaria develop into young Trematodes. Belone 

 vulgaris has finally to be devoured by a Dogfish or Ray before the 

 young worms become adult. 



II. Again the disease known in Europe as liver-rot in sheep 

 is caused by a worm {Distomum hepaticum) of the same genus as the 

 larvae described from Nandus marnioratus. The eggs from the 

 worm causing liver-rot are liberated with the faeces of the sheep, 

 and these attack a small snail {Linmea truncatula). The earlier 

 larval, sporocyst and redia stages are passed in the tissues of the 

 snail. The resulting cercaria leave the snail and encyst on the 

 grass. In this condition they are again eaten by sheep and the life 

 cycle recommences. In the case of Distoniiun hepaticum the earlier 

 larval, sporocyst, redia, and cercaria stages all develop in one host, 

 whilst in the case of the Gasterostomum already cited, the preceding 

 stages are distributed between two hosts, namely mussels and the 

 Gar fish. 



In the case of the young worms under consideration, the earlier 

 larval, sporocyst, redia and cercaria stages have already been 

 passed through. These stages, or part of them, are possibly passed 

 in the snail Vivipara bengalensis which occurs abundantly along 

 the banks of most rivers in Northern India. These must needs be 

 eaten by the fish Nandus marmoratus before the young forms 

 develop. • The young worms are apparently liberated with the 

 ripe eggs of the fish. These eggs are extensively devoured by frogs 

 and voracious fish, and it seems Hkely that the adult worm 

 occurs in such an host. A limited investigation of about 60 

 frogs found on the banks of the Bhagirathi yielded no result, and 

 up to the present, other probable hosts have not been examined. 

 hiiiton' s yonng Distomum , referred to above, was apparently from 



