304 



NATURAL HISTORY, 



THE TRUE FISHES. 



One of the most marked peculiarities of science is the way in which 

 the meaning of terms is changed, and their use restricted with our advances 

 of knowledge. In years gone by the term fishes was used to indicate 

 almost everything living in the sea, and even to-day the terms shell-fish 

 and star-fish remain in our vocabulary, although every one knows that they 

 are not fishes at all in the proper sense of the word. More lately the same 

 term has undergone a still farther restriction by scientific men ; for while 

 some forms, like the lampreys, are as fish-like in appearance as are the 

 eels, in their structure and mode of growth they are as different from a 

 codfish or a perch as these latter are from a snake or turtle. So to-day 

 the term is restricted to the forms enumerated here and their immediate 



relatives ; it may be 

 in the future that still 

 further modifications 

 may be introduced 

 with our advance of 

 knowledge. 



The typical form 

 of a fish hardly needs 

 description. It is long 

 and slender, with va- 

 rious fins and a gill- 



Fig. 294. — Fish, showing the various fins: d, dorsal; c, caudal; a, anal; _„.„„., „„ ^; + l-w^ oiAa 



v, ventral; p, pectoral. C0\ ei Oil eitliei Side 



of the neck. The fins 

 which correspond to our arms are called the pectorals ; those to our legs, 

 the ventral s ; while those in the median line are the dorsal, above ; anal, 

 below ; and the caudal or tail-fin, at the end of the body. These terms are 

 rather more technical than most that we have used, but they are so neces- 

 sary in the description of fishes that we have introduced them here. 



Sturgeons and Gar-pikes. 



There are two groups of true fishes, one embracing almost all our 

 common forms ; the other, represented by the sturgeons, gar-pikes, and a 

 few other existing forms, seems on the road to extinction. Its members 

 were very numerous in geological time, and the term 'age of fishes,' applied 

 to the Devonian period of the earth's history, was due to the prevalence 

 of these forms in those ancient seas. To-day forty species embrace the 



