326 
Entozoa of Fishes 
round, with entire margin. The first modification of this is apparent 
in Podocotyle with its characteristic trilobate ovary. In Cainocreadium 
the lobes, still three in number, are more pronounced, while finally in 
Helicometra the number may be increased to five. The increasing 
distribution of the yolk-glands can be traced very distinctly from 
Allocreadium through Podocotyle and Lebouria to Peracreadium in 
which it attains its maximum. Under normal circumstances Podocotyle 
agrees with Allocreadium in having the yolk-glands limited by the 
level of the ventral sucker, but as I have shown (1909 a, p. 452), in 
certain specimens of Podocotyle atomon (Rud.) an asymmetrical group 
of follicles makes its appearance in front of the ventral sucker on the 
right side. Exactly the reverse condition will be shown later to occur 
in a species of Lebouria where the yolk-glands normally extend in front 
of the ventral sucker, but in some cases are entirely absent on one side 
in front of the sucker. The eggs in most Allocreadiinae show a 
distinct thickening of the shell at the anopercular end. Occasionally 
this may be so marked as to form a slight knob, but it is only in 
Lebouria that this assumes any size and becomes at all frequent. In 
many cases an unmistakable spine is formed, but this is by no means 
constant, for a large number of the ova have no spines and even hardly 
a trace of a knob. That the condition in Lebouria , however, is the 
forerunner of the filaments in Helicometra there can hardly be any 
doubt, unless, perchance, evolution has proceeded in the reverse 
direction. 
There is thus no direct line of development through these six 
genera and the facts seem to point to their origin from a hypothetical 
common type, closely resembling Allocreadium. This supposition is 
strengthened by the probability that it was from some such form that 
the nearly-related sub-families Lepocreadiinae, Stephanochasminae, and 
possibly Echinostominae, also arose. The position of the testes and 
ovary and the uniform restriction of the yolk-glands behind the ventral 
sucker (except in a few genera of Echinostominae ) ai’e remarkably 
constant throughout these sub-families. 
Genus i. Peracreadium Nicoll 1909. 
Species 1 . Peracreadium genu (Rud. 1819). 
This species was found frequently in considerable numbers in the 
rectum of Labrus berggylta and usually in association with Helicometra 
