444 
Marine Cereariae 
Echinostomum secundum Nicoll (i. 1906, p. 151, iii. 1906, p. 515), 
(Lebour, 1905, p. 1, 1906, p. 3, and 1908 c, p. 354.) 
(PI. xxvni, figs. 11-14.) 
Occurs in the digestive gland of the common periwinkle Littorina 
littorea from Budle and Fenham Flats in about three per cent, in spring, 
summer, and early autumn, also in about five per cent, from Loch R 3 'an. 
The spire of the animal is always a brilliant orange and can easily be 
distinguished from healthy specimens which are not infected. The 
colour is due to the redia which is a bright pinkish orange when full 
grown. The very young rediae are colourless, the smallest seen was 
0‘40 mm. long, with a conspicuous pharynx and intestine, collar, and 
two ambulatory processes posteriorly. The anterior part between 
mouth and collar is grooved. The collar and grooves disappear and 
the ambulatory processes gradually dwindle in the full-grown redia, 
but there is always a trace of one of the processes, the redia having 
the shape of a stocking; it may be 2 or more mm. long, and is much 
more constant in shape than E. leptosomum. 
The cercaria is colourless and transparent, length O’TO mm. without 
tail which is not quite so long aa the body. Body covered for the first 
two-thirds with small spines. Head armed with 29 spines arranged as 
in E. leptosomum but with the two short spines at each end, which are 
on a lower level, much shorter than the others. Oral sucker 0 065 mm. 
across, leading to a thin prepharynx, pharynx 0'05 x 0‘03 mm., narrow 
oesophagus with intestinal caeca reaching nearly to the posterior end of 
body. Ventral sucker 0 095 mm. across. Excretory vesicle oval, two 
much branched lateral canals full of clear granules. Cystogenous ducts 
opening by oral sucker. 
The cercaria leaves its first host and swims freely in the water until 
it reaches its intermediate host, which in this case is the common 
mussel Mytilus edulis in which it settles down and encysts in the foot. 
I have determined by experimental infection that this cercaria passes 
from the periwinkle to the mussel and encysts in the foot of the 
mussel (1909, p. 353). Cysts which are almost identical with these 
and probably the same species wei’e also found in Cardium edule, 
Mya arenaria and Tapes pidlastra. The cysts project from the foot 
in well-infected specimens as small papillae dotted all over it. Each 
cyst measures 0'21-0‘25 mm. across, and, except for the size, is exactly 
