508 TRANSACTIONS OF SECTION D. 
usually in very considerable numbers. Sometimes three or four larve were 
found in one cyst, and in some cases the larve had emerged and become free in 
the body-cavity of the trout. The larve varied in Jength from a quarter of an 
inch to eight inches. They were about a quarter of an inch broad in the largest 
forms, and very thick dorso-ventrally. While alive they have the appearance of 
being segmented, and might easily be mistaken for adult tapeworms. Sections 
showed that the sexual organs had not yet begun to develop. 
The infection is apparently very general in this particular loch, a considerable 
number of the trout fished being parasitised to a greater or lesser extent, and 
in some cases the fish become considerably emaciated as a consequence, and 
their dead bodies have been observed in the river which issues from the loch. 
Endeavours have been made to ascertain whether a similar disease exists in 
any other part of the country, but so far the information supplied tends to lead 
me to a negative conclusion for those places from which the information has 
come, with the one doubtful exception of the lochs of Ross-shire and Suther- 
landshire. There can be little doubt, however, that the parasite will eventually 
be found in other localities. 
We now come to the question of the life-history. What is the*source of 
infection, which has, I understand, increased much of late years? From the 
nature of the parasite it was clear that the sexual form (tapeworm) must be 
searched for in some vertebrate frequenting the loch, and suspicion naturally 
pointed to a bird as being the most probable host, for there are no mammals, 
with the exception of the rat, which could serve the purpose of harbouring the 
adult worm. Of the water-birds frequenting the loch, I have examined three 
gulls, two of the species Larus canus and one Z. fuscus, and also a number of 
adult and young merganser (Jf. serrator). The only cestode parasites found in 
the former birds were a number of undetermined Cyclophyllidew, which bore no 
relation to the larva from the trout. One of the adult merganser examined had 
in its intestine an adult sexual tapeworm in large numbers, and a consideration 
of the anatomy of this form led to the view that this was possibly the adult I 
was seeking. The parasite was an undetermined species of Diphyllobothrium. 
Unfortunately the sex of the particular bird harbouring the parasite was not 
noted, but all the other adult geese examined were females, and it is possible 
that these had lost their parasites during incubation, and would regain them 
later. It will be one of the objects of future research to test the validity of this 
view, which I owe to Mr. Caldwell. I may mention that this year’s young are 
without any cestode except the Schistocephalus now to be referred to. 
In addition to the isolated instance of the occurrence of Diphyllobothrium, 
sp., all the merganser were found to be infected with the larval and adult 
sexual form of Schistocephalus gasterostei (Fabr.), obtained from the stickle- 
back, which forms a large part of the food of these birds. This worm lives in 
the asexual form in the body-cavity of the fish, and, as is well known, attains 
maturity in the intestine of the bird, which it leaves per anum in the course of 
a few days after infection. The mature worm has been observed living free in 
the water. 
That the Diphyllobothrium found in the intestine of the merganser is the 
sexual stage of the larva of the trout is by no means proved, but the evidence— 
i.e., the anatomical features, based principally upon the size and shape of the 
head, and the appearance and distribution of the suckers; the presence of the 
worm in great quantities in one bird, and the difficulty of suggesting any other 
possible host—is very strong. ‘The problem can only be definitely elucidated by 
means of infection experiments, which are now being performed, but the results 
are not yet known. 
It has been suggested that the trout itself may harbour the adult, that the 
larger trout eat the smaller, and so take the larve into the digestive tract, 
where they attain maturity. It is true that an adult tapeworm, Abothrium 
crassum (Bloch) = Bothriocephalus infundibuliformis (Rud.), the life-history of 
which is unknown, is present in the alimentary canal of Salmo fario, sometimes 
in considerable numbers, but to judge from the size and shape of the scolex, 
and the distribution of the suckers, I do not think it is the sexual form of the 
plerocercoid I have described. I have also obtained Abothrium crassum from 
trout from localities where the plerocercoid is unknown. 
