394 WILLIAM NICOLL. 
uterus and ova, which, as Odhner has in part indicated, and 
as I have tried to show, are really the generic features dis- 
tinguishing Fellodistomum from Steringophorus. 
Amongst the various histological features to which attention 
has been paid are the size of the muscle-fibres and of the 
elements of cellular structures. These, however, depending 
as they do so much on the state of contraction and the 
method of preservation, can hardly be considered of much 
practical utility. Other features of more importance which 
have been dealt with are the nature of the yolk-gland 
secretion (see under Stephanophiala laureata), the intes- 
tinal epithelium, and the cellular elements of the suckers in 
Fellodistomum fellis. 
Reference may be made here to an interesting bionomic 
problem, which unfortunately does not admit of easy 
solution, namely, the length of time required for the cercaria 
to attain maturity on entering its final host. It is easily 
ascertainable that it does not commence to produce ova, 
which may be regarded as the criterion of maturity, im- 
mediately on entering its host. A longer or shorter period of 
time usually elapses, during which it increases considerably 
in size. ‘I'he question has occurred to me in connection with 
various species, and here particularly in the case of Lebouria 
idonea. It has been a matter of frequent observation that 
a certain definite size limit exists between immature and 
mature specimens, or, in other words, that egg-production 
begins at a more or less constant period. In the case of 
Lebouria idonea the smallest mature specimen measured 
1-6 mm. and the largest immature specimen was of the same 
length. Again, in Podocotyle atomon the limit of matu- 
rity appears to le somewhere between ‘9 mm. and 1:0 mm. 
Many other examples might be given, but these will serve as 
illustrations. ‘The smallest individuals found in the above 
two cases measured respectively 1°0 and 65 mm. _ It is fairly 
certain, however, that the cercarie in each case are some- 
what smaller than the above minimum limits, but taking 
them at those lengths it is evident that the cercarize must 
