FISHES OF THE BRITISH ISLANDS. 



Our ancestors were accustomed to call by the name of fish 

 all the creatures which inhabit the waters; and in so doing they 

 comprised under this term all the races of crabs and lobsters, 

 and also many species of shell-fish, as oysters and cockles. It 

 was even a disputed point among them whether the otter should 

 not come under this denomination; to which this animal must 

 be admitted to have as good a right as the bat to be classed 

 among birds; among which, simply on account of its powers 

 of flying, it continued to hold a place to even a modern date. 

 But a better knowledge of nature has corrected these mistakes, 

 and we limit our subject to creatures pointed out by the following 

 characters. Not only, therefore, do we say with Dr. Monro, 

 in his work on the structure and physiology of fishes, that by 

 this name we understand that class of animals which lives in 

 water, swims by the assistance of fins, and has the water directly 

 applied to the gills, through which organ the whole mass of 

 blood in the body passes in the course of circulation: which 

 definition is so far deficient, that it would not exclude the 

 young condition of the several kinds of frogs and newts : but 

 we add also, that they are furnished with nostrils, usually double 

 on each side, which do not communicate with the mouth or that 

 passage by which they receive the water which passes through 

 the gills. In a fish also the whole mass of blood passes through 

 the gills for the purpose of receiving the influence of air con- 

 tained in the water, without being again returned to the heart 

 until it has been carried to the other parts of the body. This 

 last observation is probably referred to by Monro, but is not 

 fully expressed by him, and in these particulars all fishes agree ; 

 but there are other characters among them which are sufficiently 

 distinct in different families as to render it necessary for us to 

 divide them into classes; of which, for reasons presently to be 

 assigned, we shall place the Sharks and Bays at the head: in 

 doing which we are not singular. The illustrious naturalists, 

 Owen and Agassiz, have done the same; and Linnaeus, whose 

 VOL. I. B 



