PERIODICAL MIGRATIONS OP FISHES. 239 



action of the water in which it resides, and it is ob- 

 served that those species in which the scales are 

 small, are furnished with the largest quantity of pro- 

 tecting mucus or slime. 



The membranes of the fins are thin, and more or 

 less transparent, supported by slender, elongated por- 

 tions of bone, of variable degrees of flexibility. The 

 names given to the different fins have reference to 

 the particular part of the body to which they are 

 attached. The principal organ of motion in fishes 

 is the tail, assisted by the simultaneous action of 

 the pectoral and ventral fins. The economy of Na- 

 ture 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 are found to belong to each of 

 the species, thus affecting by choice these different 

 stations in the water. 



By a portion of the older Ichthyological writers, 

 fishes were supposed to perform, like some birds, 

 certain periodical migrations, making long voyages 

 from north to south at one season of the year, and 

 the reverse at another. It does not seem to have 

 been sufficiently considered, that, inhabiting a medium 

 which varied but little either in its temperature or 

 productions, locally, fishes are removed beyond the 

 influence of the two principal causes which render a 

 temporary change of situation necessary. The law 

 of Nature which obliges them to repair to the shal- 

 lower water of the shores at a particular season, ap- 



