2 SALT-WATER FISHES 



be defined as a class on the basis of their manner of repro- 

 ducing their kind. The majority of our fishes, it is true, 

 accomplish this, like frogs, by depositing spawn ; on the other 

 hand, several of them, belonging to at least three widely 

 separated groups, bring forth the young alive. Even the 

 residence in salt water and the manner of breathing with 

 the aid of gills are not sufficient, either separately or in conjunc- 

 tion, to distinguish the fishes from other classes of animals. 

 The whale, the porpoise, and the seal live in the sea, but they 

 are warm-blooded mammals, and they breath the air direct 

 with lungs. The lobster and shrimp, which are crustaceans, 

 and the oyster and the squid, which are molluscs, live in the 

 sea and breathe with gills, but they have no backbone. 

 Neither have the anemones and medusae, and not even the 

 vernacular use of the word " fish " in such compounds as star- 

 fish, jelly-fish, or shell-fish, deceives many people in these days 

 of universal knowledge. The last so-called " fish " to retain 

 their hold as such on the popular belief were the lamprey and 

 hag-fish, and indeed it is with some hesitation that the writer 

 has excluded them from this volume. At the same time, it was 

 felt that they were fish-like rather than true fishes, and that 

 including them would have involved a detailed explanation of 

 anatomical divergence unsuited to the scope of these pages. 



If we desire to select some character that shall be constant 

 in all fishes, from no matter what portion of the globe, from 

 fresh waters or salt, we are restricted — -so far, at any rate, as 

 external features go — to the organs of locomotion known as 

 fins, otherwise the limbs of the fish. 



The ceratodus of Australia, with allies in South America 

 and Central Africa, has a development of the air-bladder which 

 is virtually a lung for use at those dry periods of the tropical 

 year when water fails and the gill-breathing apparatus would be 

 ineffectual. The common conger-eel has no scales. India and 

 other countries contribute to our museums fishes that were 

 during life capable of living for considerable periods out of 



