10 BRITISH FRESH-WATER FISHES 
ingress into certain areas of such fish as cannot survive some- 
what prolonged exposure to the air. To meet this difficulty, 
as will be shown later, the common eel possesses a peculiar 
formation of gill-cavity, which retains enough moisture to 
preserve the activity of the branchial /amine so long as 
to enable the animal to perform considerable journeys 
overland. 
The subaqueous life of fishes has led to the modification of 
their two pairs of limbs into fins, although the precise analogy 
of these organs to the legs of terrestrial mammals and reptiles 
may easily be lost sight of in certain genera and species by the 
development of unpaired, vertical, or median fins, which some- 
times eclipse in size and prominence the original limbs, as in 
the perch (Fig. I., 75). Sometimes all external trace of the 
original limbs is lost, as in the eel, which has no ventral fins 
representing the hind legs, but has developed instead a con- 
tinuous, confluent median fin, extending longitudinally over 
the greater part of both upper and lower surfaces. The total 
disappearance in the eel of those limbs which perform the 
office of propulsion in terrestrial animals is the more remark- 
able when it is considered that, of all British fishes, this is the 
only one that ever voluntarily leaves the water to travel by 
land. The eel, however, retains fully-developed pectoral fins, 
the homologues of arms in man. 
Ray and Willughby, two excellent naturalists and collabo- 
rators of the seventeenth century, arrived at a definition of 
fishes which we recognise as perfectly sound and complete at 
this day. In the class of Fishes they placed all animals with 
blood, breathing by gills, provided with a single ventricle of 
the heart, and wholly or partly covered with scales, or naked. 
But the exclusion of whales from the class of Fishes involved 
in this definition proved so startling and unacceptable to the 
scientific men of the day, that Ray afterwards erroneously 
modified his terms so as to include warm-blooded animals 
inhabiting the water. 
