SHAllP-NOSED EEL. 287 



1 am, however, of opinion, that the passage of adult Eels to 

 the sea, or rather to the brackish water of the estuary, is 

 an exercise of choice, and not a matter of necessity ; and 

 that the parent Eels return up the river as well as the fry." 



" All authors agree that Eels are extremely averse to 

 cold. There are no Eels in the arctic regions, — none in 

 the rivers of Siberia, the Wolga, the Danube, or any of its 

 tributary streams ; yet the rivers of the southern parts of 

 Europe produce four species. There is no doubt that fishes 

 in general, and Eels in particular, are able to appreciate 

 even minute alterations in the temperature of the water they 

 inhabit. The mixed water they seek to remain in during 

 the colder months of the year is of a higher temperature than 

 the pure fresh water of the river, or that of the sea. It is 

 a well-known law in chemistry, that when two fluids of dif- 

 ferent densities come in contact, the temperature of the mix- 

 ture is elevated for a time in proportion to the difference in 

 density of the two fluids, from the mutual penetration and 

 condensation. Such a mixture is constantly taking place at 

 the mouths of rivers that run into the sea, and the mixed 

 water maintains a temperature two degrees warmer than that 

 of the river or the sea. This elevation in the temperature of 

 the water of estuaries and the mouths of rivers is, I have no 

 doubt, one reason why they in general abound in young fish." 



In a tideway river the descent of the Eels towards the 

 brackish water takes place during the autumn, and various 

 devices are employed in diflferent streams to intercept them 

 in their progress. The vignette at the bottom of the next 

 page represents the form of an apparatus used in various parts 

 of the Thames, called an Eelbuck, consisting of a framework 

 of wood supporting various wicker-baskets of a particular 

 form. The large open end of each basket is opposed to 

 the stream, and by the peculiar structure of the inside, 

 any fish once within the body of the basket, cannot escape. 



