M. V. Lebour 
3 
E. leptosonium in Scrobicularia tenuis). The cercaria is developed in 
long rediae very like those of Echinostovmm and its general form, 
suckers, alimentary canal, excretory system, tail and head spines 
(although the latter are somewhat different in form) remind us of this 
group. The excretory vesicle however does not extend into the tail, 
which is solid except for a sucker-like structure at the hinder end 
(Lebour 1912, pp. 416-7, Figs. 1-3, Plate XXIX). From these notes 
there seems to exist a close relation between Parorchis and the 
Echmostojnwn group, as is pointed out by Odhner who regards the genus 
as an aberrant form of an Echinostomid (Odhner 1913). 
The length of a full-sized cercaria before it emerges from its first 
host is 0'64 mm. It has, when of this size, large spines on its body and 
spines round its head. It may be found, when full grown, either free in 
the digestive gland having come out of the redia, or in the mantle cavity 
near the gill, having migrated from the digestive gland on its way out 
of the body. This is as far as I can follow it until we find it again in the 
cloaca of the Herring Gull. The smallest worm from the cloaca measured 
2’4 mm. (Plate I, fig. 1), so that a large increase in growth has taken 
place. The form of the body is identical, the body covered with thick 
spines dwindling towards the posterior end and a crown of spines 
forming a sort of collar interrupted ventrally by the oral sucker. The 
alimentary canal has the same form and likewise the excretory vesicle 
with its lateral branches, which however are much more conspicuous 
and complicated than in the tailed cercaria. The suckers have both 
increased in size especially the ventral sucker. In the smallest the oral 
sucker measured ()T6 mm. and the ventral 0'40 mm. (oral sucker and 
ventral sucker of tailed cercaria from Purpura 0'06 mm. and 0'09 mm. 
respectively). In larger forms the increase is greater. In specimens 
2’8 mm. long the oral sucker was 0‘32 mm. and ventral sucker 0'60 mm. 
across. Size in mature worm, oral sucker 0'50 mm., ventral sucker 
usually more than twice as great (Nicoll). 
It is interesting to find this intermediate form as it indicates the 
first stage in the life history of Paroi'chis acanthus and also shows that 
at any rate in its early life it is exceedingly like EcJdnostoynum. I have 
included it in this group when trying to classify the cercariae and these 
young stages help greatly in the elucidation of the relationships and 
proper systematic position of the adults. 
1—2 
