280 THE FRESH-WATKK FISHES OF EUROPE. 



rapidly adapts itself 'to its surroundings. A living- Black Trout, according to 

 Mr. C. St. John, placed in a white basin becomes pale in half an hour, and in 

 some days becomes absolutely white ; and, conversely, a white fish put into a 

 black vessel, becomes in a quarter of an hour as dark-coloured as the bottom of 

 the jar, and is, therefore, almost invisible. Hence, almost every river pos- 

 sesses variations of colour peculiar to itself, and we are led to conclude that 

 colour cannot be of much value as a specific characteristic. 



Mr. Day further shows that a small race of Trout transferred to a lake 

 where food is plentiful attains a large size, thus confirming the experience of 

 the Austrian naturalists. 



Salmo stomachicus (GUNTHKR). 



98 



D. 15, A. 1218, P. 13, V. 9. Scales : lat. line 125, trans, f 



ii3. 



This Trout is limited to the loughs of Ireland, where it is known as the 

 Gillaroo. It was first observed, as Mr. Day remarks, by the Hon. D. Bar- 

 rington, in 1773, who found no exterior marks to distinguish it from the 

 Common Trout. There is a red Gillaroo, with black spots on it, and a white 

 Gillaroo, with black spots, which is smaller, and said to be better eating. 



Dr. Giinther has termed this species stomachicus, in allusion to the remark- 

 able thickening of the middle muscular layer of the stomach, which is com- 

 pared by Day to the gizzard of a bird. The stomach increases in thickness as 

 the fish reaches the adult condition; and since this Trout feeds on Limntea, 

 Ancylus, and other fresh-water mollusca, the stomach appears to have de- 

 veloped as the character of a local race the power of crushing these shells. 

 It is interesting to note Mr. Day's statement that he has seen examples 

 of the Salmo ferox from various localities with a muscular stomach ; 

 showing, apparently, that the tendency in this organ to vary is by no means 

 peculiar to the Gillaroo, though Sir Humphry Davy states that he caught 

 fish no longer than his finger, and found the stomach as hard as in the larger 

 fish, and with walls as thick in proportion to its size. He further remarks 

 that the Charr, which feed at the bottom, in the same way as the Gillaroo, 

 have a stomach of the same kind, but not quite so thick. Yet although the 

 food and conditions of nutrition have changed the stomach in this curious 

 manner, there are still forty-four pyloric appendages. 



The back and sides are marked with reticulated black spots or, according 

 to Yarrell, with dark reddish-brown spots on a yellow-brown ground. On or 

 below the lateral line there is a row of red spots. The free margins of the 

 dorsal, anal, and caudal fins are white, and the dorsal has black spots. 



