TRANSACTIONS OF SECTION D. 5O9 
Against the view that the merganser is the final host, the following facts may 
be alleged :— 
1. The larve in the trout are often considerably larger than the sexual worm 
in the merganser. As, however, little is known of the development of these 
forms, this objection may eventually prove to be of little importance. 
2. The number of mergansers hardly appears to be sufficient to infect the 
whole loch. 
The only reference that I can find in the literature to a similar parasite is one 
by Linton in the ‘Bulletin of the United States Fish Commission,’! entitled 
“A contribution to the life-history of Dibothrium eordiceps, a parasite infecting 
the trout of Yellowstone Lake.’, The author here asserts that the adult worm was 
found in the intestine of the white pelican (Pelicanus erythrorhyncus), but it is 
doubtful, from the account given, if this identification is correct. Infection 
experiments were not performed, and the figures given tend to indicate that the 
scolices of the plerocercoid and the sexual worm are somewhat different. 
Although I have as yet been unsuccessful in working out the life-history of 
this trout parasite, I have thought it worth while to lay these remarks before the 
Section, in the hope that they may elicit some information from Scottish 
naturalists and sportsmen which will lead to the solution of the problem, a 
result which is very desirable, as in Loch Morar, at any rate, the fishing has 
become greatly depreciated in late years owing to the prevalence of the parasite. 
7. Recent Progress in Helminthology. By W. Nicotn, D.Sc., M.D. 
The study of the parasitic worms is of considerable importance from two chief 
points of view, (1) because they are, as a class, the most complete and typical 
examples of the phenomenon of parasitism; (2) because of the part they play in 
diseases of man and animals. Considered also from the purely zoological stand- 
point they present many features of interest. 
During recent years the subject has made rapid advance, this being due to a 
variety of reasons, the chief of which are the fuller recognition of the part which 
these worms play in disease, the more thorough application of the experimental] 
method of investigation, and a more satisfactory method of classification. In 
the absence of thoroughly reliable means of identification experimental and 
other investigations are apt to be involved in confusion. For that reason, if for 
no other, the science as a whole is particularly indebted to workers, such as 
Looss, Fuhrmann, and Railliet, who have made successful efforts to evolve a 
natural system of classification. The most radical change introduced has been 
the substitution of internal morphology in place of external characters as the 
basis of diagnosis, except in the case of the Nematodes, in which chief stress is 
laid on the head and tail characters, although here also attempts are being made 
to utilise the internal structure. A study of the bionomics of these parasites 
affords an important criterion of the value of these methods of classification. 
Under earlier systems there appeared to be little correlation between habit and 
systematic position, a relation, however, which is clearly brought out by the 
present method. Habit, indeed, may in very numerous instances be regarded 
as a useful indication of systematic position. In the matter of development 
several contributions of revolutionary importance have been made, the most out- 
standing of which concern the life-histories of the hook-worm (Ankylostoma), 
the blood-fluke (Schistosomum), and the Guinea-worm (Dracunculus). In the 
case of the first two, it has been demonstrated that infection may and does take 
place through the unbroken skin. It is over fifteen years since Looss first pub- 
lished this discovery in the case of the hook-worm, but it aroused so much 
controversy that only within the last year can the matter be regarded as 
irrefutably proven. The fact that the Guinea-worm passes its larval life in a 
fresh-water crustacean and that infection takes place through the mouth was 
another discovery which rather upset preconceived notions. In the matter of 
morphology chief attention has been directed to the structure of the so-called 
shell-gland. Recent researches have shown that the shell substance is secreted 
1 Vol. ix., 1899, pp. 337-358. 
