FISHES OF THE BRITISH ISLANDS. 
Our ancestors were accustomed to call by the name of fi.sb 
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 
he admitted to have as good a right as the bat to he 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 
t lis 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 
t ie gills. In a fish also the whole mass of blood passes through 
the gills lor the purpose of receiving the influence of air con- 
tamed 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 dltfereiit 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 Linn^us, whose 
VOL. I. H 
