| 
} 
| Mr Saunders, A Note on the Food of Freshwater Fish. 237 
larger and tend to be carnivorous; the smaller the fish the greater 
are its carnivorous propensities. Thus one finds that in a fish of 
about 4 cm. long the stomach and intestine will be full of insect 
larvae and small crustacea, while the rectum is distended with 
_Nitzschia sigmoidea shells. In one of a smaller size the whole of 
_the alimentary canal was full of insect larvae and small crustacea 
only, or we may find insect larvae and crustacea in the stomach 
and Peridinians in the intestine. 
) Two points are worth noticing here. The first is that the fish 
does not pick up anything digestible that comes handy but it 
confines its attention to one food only, or rather to one class of food. 
The stomach never contains a mixture, but is always full of one 
thing only. The second point is that it 1s capable of eating both 
animal and vegetable food. 
As regards the animal food that is found in the stomach, it is 
a mixture of insect larvae and small crustacea of all kinds. These 
the fish evidently hunts for and catches by the use of its eyes only 
and not by the sense of smell. This may be amply proved in the 
following manner. If some sticklebacks be kept in an aquarium 
with some small waterbeetles, they will never attack the beetles, 
which are not an article of their diet. But if a number of 
Copepods are introduced into the aquarium the fish will at once 
attack them and eat them, at the same time they will often attack 
a waterbeetle by mistake, but they let the beetle go immediately, 
never swallowing 15. It is only for a moment after the Copepods 
have been introduced that the beetles get attacked, the stickle- 
back very soon learns to leave them alone. Sticklebacks will also 
snap at Copepods enclosed in a glass tube if this tube be placed in 
their aquarium. 
But as regards the vegetable food I think that the stickleback 
must hunt for this by the sense of smell. It is difficult to see how 
the fish could get a meal that entirely consists of Peridinians or 
WV. sigmoidea using only his eyes. And if he swam through the 
water with his mouth open he would surely engulf a mixture of 
organisms, and this is found to be not the case. 
But so far I have been speaking of sticklebacks taken from one 
pond only, and from an examination of the contents of the stomachs 
of specimens caught in this pond we might assume that all young 
sticklebacks of about 4 cms. long are carnivorous, while the adults 
are vegetarian. This pond of which I have just been speaking is 
one of a series of ponds which have been formed in the excavations 
made by a former brick industry. They are all very close together, 
the banks between them being not more than a few yards wide. 
Yet in one pond it is found that ail the adult sticklebacks feed 
only on a Diatom, Nitzschia sigmoidea, and in another all the 
adults are carnivorous and hunt for insect larvae and small 
VOL. XVII. PT. III. 16 
