Marine Cercariae 
(4) In Mytilus eclulis (the common mussel) 
EcJdnostornum secundum (foot) 
Gym7i02)hallus margaritae (between mantle and foot) 
(5) In Cancer pagurus (the edible crab) 
Sjyelotrema excellens (all over body). 
Systematic Section. 
I. Gastekostomata. 
Genus Gasterostomum 
Gasterostomum gracilescens (Rud.) (Lebour, 1908 B, p. 8). 
Larval form Bucephalus haimeanus Lacaze-Duthiers. 
(P). XXV, figs. 1-4.) 
This is the well-known oyster parasite. In Britain however it has 
only been observed in the cockle Cardium edule L., but always in the 
vicinity of oyster beds. It occurs in two per cent, of the cockles from 
Fenham Flats, Northumberland, and more commonly in the same 
Mollusk from Emsworth, Hampshire (in about 10 per cent, of those 
which live near the oysters). For the specimens I am indebted to 
Miss Florence Jewell of Emsworth, who kindly sent me them on several 
occasions. Johnstone (1904, p. 106) has recorded a specimen of 
Cardium edule from the Lancashire coast infested with the parasite. 
The cercaria occurs in very long sporocysts which are irregularly 
branched at intervals and so tangled together that it is almost 
impossible to separate them (fig. 1). They are packed closely together 
all over the body except the foot, completely destroying the gonad. 
Inside the sporocysts are the Bucephalus cercariae in all stages. The 
full-grown cercaria (fig. 2) measures 0'26 mm. in length without the 
tail which is extremely contractile and may be many times longer 
than the body. The tail is a very characteristic organ consisting of a 
triangular median portion at the hind end of the body bearing on each 
side a lateral process which is thin and elastic, alternately contracting 
and expanding as the cercaria swims. The body is covered, except at 
the tail, with minute sj^ines arranged in transverse rows which give it a 
striated appearance. The mouth is in the centre of the ventral sucker 
which leads to a simple sac-like intestine. At the anterior end of the 
body is a sucker-like cystogenous organ. Posteriorly is a clear pear- 
shaped excretory vesicle. 
