INTRODUCTION. Xlll 



this difference in structure. The number of fin-rays in each 

 fin of different examples of the same species of fish is not 

 always exactly alike. 



The names given to the different fins are derived from the 

 part of the body to which they are attached. The position 

 of one pair, the ventral fins, attached as they are to the an- 

 terior and sometimes to the middle of the belly, affords a 

 valuable character for distinction. These ventral fins are 

 considered analogous to the hinder limbs in other animals ; 

 the pectoral fins to those attached to the shoulder ; and many 

 points of resemblance exist in the structure. The principal 

 organ of motion in fishes is the tail, assisted by the simul- 

 taneous action of the pectoral and ventral fins. The older 

 writers on Ichthyology considered that the perpendicular po- 

 sition maintained by the fish was owing to the presence of 

 the dorsal fin ; but an experiment detailed in this volume, at 

 page 365, appears to indicate that the power of sustaining a 

 particular position in the water is due, in part at least, to 

 other causes. 



The economy of Nature is conspicuous in the habits of 

 fishes. Some always swim at or near the surface, others 

 about mid-water, and many close to the bottom : all parts of 

 the water are alike occupied, and some peculiar qualities and 

 powers being found to belong to fishes affecting by choice 

 these different stations in the water, they will be occasionally 

 referred to by the terms of surface, mid-water, and ground 

 swimmers. To include the extremes to which their powers 

 of motion are occasionally applicable, some, as the Flying- 

 fish, pages 453 and 458, are able to sustain themselves for a 

 short time in the air, from the momentum obtained by their 

 previous exertions before quitting the water ; and others, by 

 the strength of the serrated bony ray in each pectoral fin, are 

 able to transport themselves overland from one pool in search 

 of another. 



