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 Limncea 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 growth 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- 



