M. V. Lebour 
427 
exjjeriments to say which it is when so many of these closely similar 
species are common in the same host, 
The young cercaria occurs in sporocysts in Gardium edide and Tapes 
decussata. The sporocysts are simple, more or less spherical sacs, quite 
colourless, some large and some small, Jameson (1902, p. 153) found 
them in Tapes in the muscular or connective tissue of the mantle 
margin, particularly dorsally, close to the siphonal muscles. He thinks 
that they increase by budding and grow as the Mollusk grows. In 
between the sporocysts are often to be found chalky spots, the develop¬ 
ment of which Jameson regards as due to the compression of the 
tissues, owing to the growth of the sporocyst and this interferes with 
the secretion of fresh shell layers; hence these white chalky masses. 
Gardium edide was also found by Jameson to be a host for the 
sporocysts and these also occurred in the mantle margin. Afterwards 
Nicoll (l. 1906, p. 149) at St Andrews and myself (1907 A, p. 18) in 
Northumberland found these sporocysts very frequently in Gardium 
edide but not usually in the mantle margin. The usual habitat was 
just under the umbo where they occurred as a jelly-like mass frequently 
with the chalky spots mixed with them. The cockles of Fenham Flats 
and Budle Bay, Northumberland, were very commonly infected by this 
parasite. I also found it in the cockles at Millport. 
The sporocysts (fig. 7) contain the cercariae (fig. 8) which measure 
when fully formed 0T5-0'3 mm. in length, the body covered with 
spines, a pair of brown eye-spots at the anterior end, one on each side of 
the oral sucker. The ratio of the oral to the ventral sucker is about 
2:3. The lobes of the intestine do not reach so far as the ventral 
sucker and have no food inside. The testes and traces of the other 
reproductive organs can be seen in dead specimens only. The excretory 
sac is large with its lateral forks reaching as far as the pharynx. 
The cercaria presumably leaves its first host and by aid of currents 
or by crawling on the ground by aid of its suckers, reaches its 
intermediate host, Mytilus edtdis. Here it settles down between the 
mantle and shell, feeds and grows and finally makes its way into 
the tissues of the mantle, curls itself up and rests. It does not itself 
secrete any cyst but the mantle cells grow round it and form an 
enclosing sac of epithelium. Jameson believes that the parasite 
stimulates the mantle parenchyma to form these epithelial-like cells 
which are capable of secreting a pearly-layer round the parasite. The 
cercaria may make its escape from the sac before the pearly-layer is 
formed otherwise it dies forming a nucleus for the pearl. 
28—2 
