516 M. V. LEBOUR. 
developed compact oval masses of cells with large nuclei. Ovary and 
vitellaria are not as yet differentiated, although masses of nuclei probably 
represent these in the process of formation. 
The excretory vesicle is an oblong sac with very thick walls composed 
of large cells. It is conspicuous at the hind end of the body reaching to 
about the level of the posterior margin of the testes and opening at the 
extreme hind end in a small papilla. 
These features show it to be very like the structure of Zoogonus vivi- 
parus (see Lebour, 1908), allowing for growth and development especially 
of the region behind the ventral sucker and of the reproductive organs. 
The fact also that it is the only really common fish Trematode of these 
parts with such short intestinal diverticula supports the view. The 
relationship of this cercaria to the stumpy-tailed forms seems obvious, 
the stumpy tail in this case being replaced by the peculiar sucker-like 
disc. The thick-walled excretory vesicle is common to this species and 
to all in the group and also the boring spine and glands. Except for the 
peculiarly modified hind end it fits very well into Dollfus’ group (1914) of 
Cotylocercous cercariew, which are all developed in sporocysts in marine 
gastropods. None of their life histories are so far known. 
[ have to thank my colleague, Miss G. E. Webb, for making the sections 
which were used in working out the structure of the cercaria in order to 
determine points not easily seen in the living material. 
TREMATODES IN POST-LARVAL FISH. 
Whilst investigating the food of young fish a number of Trematodes 
were found. Some of these were immature, others adult and containing 
ova. Those most frequently found were Derogenes varicus and Pharyn- 
gora bacillaris. Derogenes varicus is a common parasite of many fish, 
notably the Pleuronectids, and in an immature state was found in several 
fish, particularly Arnoglossus and Scophthalmus norvegicus. The only 
intermediate hosts so far known for this species are Sagitta and Har- 
mothoé, so it is somewhat difficult to say how the worm enters the small 
fish as they almost certainly do not eat these worms. The most likely 
explanation seems to be that by the death of the worm host the young 
Trematode is set free and is then swallowed along with other food by 
tiny fish in which it afterwards matures. 
Derogenes varicus was found in the following fish :— 
Arnoglossus sp. 23 Solea variegata 4 
Scophthalmus norvegicus 18 Gadus minutus 2 
Pleuronectes limanda il Gadus merlangus 1 
Pleuronectes microcephalus 4 Callionymus lyra 1 
