20 
is a globular mass, smaller than the testes, lying between the 
foremost testes and the intestine, slightly on the right. A short 
oviduct arises posteriorly from the ovary and gives off a Laurer's 
canal, then curves forward and receives the vitellarian duct and 
the shell gland, after which it merges into the much convoluted 
uterus, the windings of which fill almost the whole of the posterior 
two-thirds of the body. The uterus finally runs down and opens 
into the genital sinus by the side of the male duct on the right. 
The eggs are exceedingly numerous, and golden brown in colour 
(0-02C mm. long). The vitellaria form a string of fairly large 
follicles at the anterior end, one on each side, extending from 
behind the. anterior sucker to the posterior level of the intestine. 
The duct from each side curves in and joins with its fellow in a 
reservoir to the right of the ovary, which gives off the duct 
mentioned above. 
This is one of the few Trematodes whose life history is known, 
though to a certain extent only, for we have no positive proof 
(although the evidence is very strong) that the different stages 
known belong to the same worm. The curious cercaria Bucephalus 
hmmeanus, Lacaze Duthiers, is believed to be the larval form of 
O. gracilescms, although this has not been definitely proved. A 
well-known parasite of the oyster, this cercaria has also been 
found in the cockles Cardium rmticum and Cardium edule. I have 
found it on the Northumberland coast in Cardium edule* from the 
Mussel Scaup, Holy Island. Two specimens of C cdule were found 
infested out of 100 examined. The whole of the animal in each 
case was riddled with the long thread-like sporocysts full of 
Bucephalus in various stages. As Bucephalus haimeanus certainly 
grows into a Gasterostomum, it seems most likely that it should 
grow into (?. gracUescens, which is the only member of the genus 
known on this coast. 
Dr. David Hilt Tennantf has conducted feeding experiments 
With the American Bucephalus haimeanus, and succeeded in infecting 
some fish with them. I cannot help thinking, however, that the 
adult Trematode which he provisionally calls Gasterostomum araci- 
tescms is different from the British species. Dr. Tennant is of the 
opinion that most of the species of Gasterostomum described are 
Physiological varieties of one form. It is impossible to say that 
^^i^St^^^^ Cast," by .howrhor. Trans. Nat. Hist. 
