10 
Larval Trematodes 
the middle third and posterior third of the body, and 0 23 mm. from the 
intestinal fork. It measures 0‘04 mm. across. Just in front of it the 
oviduct runs up for about 0‘02 mm. beyond it. 
The excretory vesicle is Y shaped, opening at the posterior end of 
the body. The stalk measures OTl mm. in length, each fork measuring 
0'05 mm. The vesicle also contains a refractive substance which is very 
conspicuous in the encysted worm. 
The testes are oval and occur obliquely one on each side of the 
excretory vesicle, the left in front of the right. Each measures 
0 04 mm. x 0 02 mm. 
It is evident from the arrangement of the organs, especially the 
excretory vesicle and the testes, that this is a species of Opisthorcliis as, 
so far as the anatomy can be made out, it agrees very well with the 
diagnosis of that genus (Looss 1899, p. 564), although it is described as 
without spines except in one species. It is interesting to note that the 
encysted worm found by Nicoll in the eyes etc. of Cottiis, although not 
fully described, agrees in all the points he mentions with this worm. 
The final host will in all pi’obability be found to be a fish-eating bird. 
The group to which Opisthorcliis belongs includes forms from mammals, 
birds and fishes, chiefly birds. Odhner (T912) describes an Opisthorchis 
from the gall bladder of the Electric Eel {Gymnarchus niloticus). 
Braun (1902) describes one from a Heron {Ardea stellaris) from Brazil 
which is much like the present worm. So far as I am aware no adult 
Opisthorchis has as yet been recorded from Britain and this is probably 
a new species. It seems to be the first record of a larval form of this 
genus and is interesting on that account. 
DESCRIPTION OF PLATE I. 
The following letters apply to all the figures : 
C. pigment 
C.S. cirrus sac 
E.V. excretory vesicle 
G.P. genital pore 
I. intestine 
O.S. oral sucker 
CES. oesophagus 
OD. oviduct 
O. ovary 
P. pharynx 
PPH. prepharynx 
T. testis 
V. vitellaria 
V.S. ventral sucker 
X. worm in cyst 
Figs. 1, 3, 6 and 8 are on the same scale. 
Pig. 1. Young stage of Parorchis acanthus Nicoll, from cloaca of Herring Gull. Size , 
2'4 mm. 
Fig. 2. Cercaria ligiae sp. inq., larva pressed out of cyst. Size 0-65 mm. 
