io 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 lamina 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. 



