396 WILLIAM NICOLL. 
many cases, as easily proved by later research, to specific 
distinction, have been attributed to the different. environ- 
mental conditions in different hosts. That the variation in 
such cases 1s to be explained by the environment is not at all 
certain, and it is very probable that on careful examination 
the same amount of variation would be found in specimens 
collected from the same host. Certain features are capable 
of varying within comparatively wide limits and can be 
measured with sufficient precision, e.g. the size of the ova 
and the extent of the yolk-glands. A case in point is that of 
Podocotyle atomon (Rud.), which occurs in a large 
number of different fish. In this species the ova were found 
to vary considerably in size, and a list was given! of the size 
of the ova in specimens from four different hosts, which 
apparently showed that there were different limits of variation 
in each case. ‘These measurements included a fair number of 
examples, but not nearly sufficient to determine the utmost 
limits in each host. One thing the list does show, however, 
is that in no particular species of fish are the ova distinct in 
size; such a variation, otherwise, would be sufficient to 
indicate specific distinction. J have since found in a single 
specimen of Gastrea spinachia a species of Podocotyle 
with ova greatly exceeding those from the fish already 
referred to and yet not differing in other respects from 
Podocotyle atomon. It is difficult, on this account, to 
say what effect, if any, environment may have on Distomid 
parasites, or whether the observed variations are purely the 
result of fortuitous circumstances. The question appears 
worth solving, and I hope to be able to publish later some 
observations on the same species found in numerous fishes 
from the west coast of Scotland. 
The following is a list of the species mentioned in this 
paper, with their hosts and habitat: 
'“ Entozoa of British Marine Fishes,” in ‘Annals and Mag. Nat. 
Hist.” (7)-xix,p. 76. 
