How to obtain it. 2 1 9 



foregoing, but heavier in the shell, and they seem to thrive in 

 rougher water than the Anadonta. They often furnish very fine 

 pearls, especially the Unio margaritifer, which at one time was 

 our principal source of pearl supply in this country. 



The Cycladidce, and the Pisidia are small cockle-like bivalves, 

 varying in size from three-quarters to one-sixteenth of an inch in 

 diameter. Trout eat them greedily, and we well remember one 

 large reservoir where the rapid growth 

 and fine condition of the trout was 

 ascribed to the abundance of this food 

 supply, and the trout that were caught 

 certainly had a great number in their 

 stomachs. The food of all snails con- 



Fig. 4 o. Cycias cornea. sists of vegetable growths and decaying 

 matter. They are excellent scavengers, 



and destroy no trout food, so that the addition to the other inhabitants 

 of a pond is a clear gain. With the exception of the Paludina they 

 are all oviparous, and multiply very fast under favourable con- 

 ditions. There is just one slight danger with regard to bivalves, 

 and that is the fact that they feed with their tongues out ; small 

 fish occasionally seize hold of these and get their noses drawn into 

 the grip of the shells. They have occasionally been seen swimming 

 about with one of the Cycladidce firmly adhering to them, having 

 been stoutly gripped by the nose as a reward for curiosity. 

 Occasionally, even unwary birds and animals fall victims, and on 

 one occasion so large a bird as a water hen (Gallinula choloropus) 

 was found dead from having attacked a swan mussel (A. Cygnea). 

 The beak of the bird was firmly held as in a vice, and the huge 

 bivalve being too heavy to carry about, the bird was simply 

 drowned. 



It is well known to most people that the water contains 

 animals which are parasitic on fish. Probably few persons, how- 

 ever, are aware of the great variety of these creatures. Some are 

 exceedingly obnoxious, whilst there are others again which seem 

 to be almost harmless. The sea-going or anadromous fishes are 

 attacked by parasites in the salt water, which they get rid of on 

 coming into the rivers, and vice versa, but it is chiefly with the 

 fresh water parasitic animals that attack trout that we have to 



