FOOD, HABITS, ETC. 79 
known in but a few cases. Thus, the sporocyst of 
Distomum macrostomum is found in the tentacle of 
Succinea, and completes its development in the Bird 
that swallows the Snail. The early stages of the 
Liver Fluke of Sheep (Distomum hepaticum) are 
passed in the body of Limnaa truncatula, which 
lives on the margins of ditches and small streams, 
and after floods may be left stranded on the sur- 
rounding herbage. Sheep feeding near such spots are 
apt to take in the Snails with their food, and so 
become infected. Still more interesting is the fact 
that the pearl of commerce has been said to owe its 
existence to the action of a Cestode larva (Tetra- 
rhynchus), which completes its life-cycle in the 
bodies of two successive kinds of Fish that prey— 
the one on the Pearl Oysters, the other on its fellow, 
as well as on the Oysters. The supposition is that, 
if the embryo worm, on forcing its way into the 
tissues of the mollusc carries with it some of the 
epithelial cells of the latter, an abnormal grdwth of 
pearl-secreting cells within the tissues of the animal 
results, and a pearl is formed, having the parasite for 
its central point. 
Some of the Nemertine Worms are also parasitic 
on Mollusca, while certain Leeches are likewise 
known to attack them. 
The relationship in the foregoing cases is un- 
doubted, but in other instances it may be merely 
a case of commensalism. Thus, several small Crus- 
