STUDIES ON THE DIGENETIC TREMATODES. 395 
increase in length by at least half before attaining sexual 
maturity. This, of course, does not hold good in every 
instance, for in some species the proportionate increase is 
smaller, while in others it is larger.! 
That Distomids, like other animals, have a fairly constant 
maturity-size might have been deduced from analogy. The 
production of ova as the test of maturity is open to obvious 
objections. It might be held that the animal is mature when 
the genitalia are developed, and that egg-production need not 
proceed immediately although the animal continues to grow. 
Against this must be put the fact of the constant line ot 
demarcation in size between specimens with ova and those 
without, and further, that a Distomid of adult size without 
ova isararity. Admitting that such a line of demarcation does 
exist, and that it is constant within close limits of variation, 
we should here have what might possibly be a valuable aid 
to diagnosis between nearly related species. As an example 
might be taken the case of the species of the genus Levin- 
seniella. I have previously described a mixture of two or 
more of these as Spelotrema feriatum n. sp. (see later). 
They were found in five different species of birds. In three of 
these the Distomids were fully mature, but in the other two, 
namely, Hematopus and Vanellus, they were quite im- 
mature, although of the same size, or even larger. I have 
examined several specimens of both these birds at different 
seasons of the year and found the parasites fairly frequently, 
but never in the mature state. The probability is that the 
latter specimens represent a species distinct from that occur- 
ring in Aigialitis and Pelidna. 
In the determination of the identity of similar parasites 
occurring in different hosts certain additional factors may 
enter, and it is to these that at several times by various authors 
variations in a particular Distomid inhabiting more than one 
final host have been ascribed. Differences of less or greater 
degree in external and internal characters, amounting in 
! Looss deals with the same matter in ‘‘ Distomen unserer Fische und 
Frésche,” ‘ Bibliotheca Zool.,’ vi (1894), p. 240. 
