354 Echinostomum secundum, Nicoll 
full of highly refractive granules and are the most conspicuous organs 
in the worm. 
A great part of the body is taken up by gland cells. Two straight 
ducts run forward from these, one on each side of the oral sucker and 
split anteriorly into smaller ducts which open just in front of the 
oral sucker. These glands are probably used for the secretion of the 
cyst since they disappear in the encysted condition (Plate XXIV, fig. 3). 
The cercaria is now ready to leave its first host the periwinkle- 
How it gets out has not been observed. Possibly it enters the 
alimentary canal and issues from the anus; or it may bore its way 
through the tissues of its host, or the periwinkle may die and so 
liberate the cercarise. The last hypothesis is the least likely as the 
cercarige do not all grow up at the same time, sometimes only one or 
two are ready to leave the periwinkle at one time and sometimes a 
great many reach this stage at once. 
It may be presumed that after leaving the periwinkle the tailed 
cercaria swims about in the water until it finds its second host in which 
it settles down and encysts. 
The second or intermediate host is the mussel. This is practically 
proved by the experiments given below. The same encysted cercaria 
has been found in the foot of Cardium edide, Mya arenccria and Tapes 
pullastra. The above description exactly corresponds to the encysted 
cercaria except that in the latter the tail and glands have disappeared. 
The cyst is thin-walled and measures 0‘2—025 mm. across. It is 
clear and colourless and is easily burst by slight pressure. 
The encysted stage of this worm was first briefly and incompletely 
described by myself (1904) from the cockles at Budle, and later, more 
fully, by Dr W. Nicoll (1906 a) from the cockles and mussels at St 
Andrews. Afterwards it was found abundantly in both cockles and 
mussels at Budle. It occurs occasionally in the liver of Cardium edule 
and Mytilus edulis as well as in the foot; Nicoll has also observed it 
round the mantle edge of Cardium edule. 
Almost every mussel at Budle is infested with this parasite and 
about ten per cent, of the cockles. The enormous number of the 
cysts points to a common shore animal being the first host and—the 
periwinkle abounds in Budle Bay. 
The cercaria probably enters the mussel or cockle by the mouth 
and bores its way into the soft parts of the body. The presence of the 
cysts in the liver suggests this. Those in the mantle edge may perhaps 
enter through the epidermis but it seems unlikely that they penetrate 
