30 TREMATODES OF THE NORTHUMBERLAND COAST 



measures 0-05 mm. in diameter. It is much larger than the 

 ventral sucker, which is only 0*033 ^^' The latter is situated 

 behind the centre of the body, and its inner margin is 

 distinctly crenulated. The small pharynx follows the oral 

 sucker immediately, and leads into a short oesophagus slightly 

 longer than the pharynx. The intestinal lobes are broad, and 

 extend beyond the centre of the body : these are usually 

 intensely black from food material, with here and there 

 globules of some fatty or oily substance. The excretory 

 vesicle is large, its forks reaching forward in front of the 

 intestinal lobes into the region of the glands, and it opens by 

 a small posterior pore. Behind the vesicle the testes may be 

 seen as two oval masses placed symmetrically one on each 

 side of the body, but no other reproductive organs are 

 apparent. The glands in the head region are a mass of large 

 cells with prominent nuclei. They occupy nearly the whole 

 of the region in front of the intestine, and reach down the 

 sides for more than half the length of the body. They open 

 into ducts which are visible as small apertures round the oral 

 suckers. The presence of these head glands leads one to 

 expect an encysted stage of this Trematode. An encysted 

 Gymnophallus has not been found, and Jameson* discovered 

 that his ' Pearl Trematode ' did not encyst, but lay resting in 

 a sac made of the epithelial cells of the mantle of its host, the 

 Mussel, this apparently taking the place of an encysted stage. 

 The presence of the other Gymiiophallus cercarise in the same 

 position in bivalves {i.e., between the shell and the mantle) 

 suggests that the same state of things exists in these cases, 

 and that they do not, strictly speaking, encyst. The present 

 cercaria is therefore unlike the other species of the genus in 

 its habitat as well as in the presence of these glands, very 

 probably also in having an encysted stage. It does not 

 appear to agree with any known adult form. Its nearest ally 

 (certainly its nearest in size) is probably Gymnophallus 

 somatericB (Levinson), which lives in the intestine of the 

 Eider Duck Somateria mollissima. The relative sizes of the 



* Op. cit. 



