Flow to obtain tt. 219 
foregoing, but heavier in the shell, and they seem to thrive in 
rougher water than the dzadonta. 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 Cyreladida and the Pis¢dia 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). Cyclas cornea sists of vegetable growths and decaying 
matter. They are excellent scavengers, 
and destroy no trout food, sothat the addition to the otherinhabitants 
ofa pondisa clear gain. With the exception of the Pa/udina 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 Cyc/adida 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 (Gal/inula choloropus) 
was found dead from having attacked a swan mussel (4. crea). 
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 vce versa, but it is chiefly with the 
fresh water parasitic animals that attack trout that we have to 
