190 



THE INHABITANTS OF THE SEA. 



earth.; Those fishes which are destined to live at the bottom of 

 the sea or to conceal themselves in the mud, such as eels and 

 skates, have either no air-bladder or a very small one — for 

 economical Nature gives none of her creatures any organ that 

 would be useless to them. Even the slimy glutinous matter 

 which is secreted from the pores of most fishes, and lubricates 

 their bodies, assists them in gliding through the waters, so that 

 no means have been neglected to promote the rapidity of their 

 movements. 



The skin of fishes is but seldom naked ; in most species it is 

 covered with scales, that sometimes appear in the form of 

 osseous plates, as in the ostracions, or project into formidable 

 prickles, as in the porcupine-fish, but generally offer the aspect 

 of thin laminae, overlapping each other like the tiles of a roof, 

 and embedded, like our nails, in furrows of the skin. In nearly 

 all the existing fishes, the scales are flexible and generally either 

 of a more or less circular form (cycloid), as in the salmon, 

 herring, roach, &c, or provided with comb-like teeth projecting 

 from the posterior margin (ctenoid), as in the sole, perch, pike, 

 &c. ; while the majority of fossil fishes were decked with hard 

 bony scales, either rhomboidal in their form, of a highly 

 polished surface, as in our sturgeons (ganoid), and arranged in 

 regular rows, the posterior edges of each slightly overlapping 

 the anterior ones of the next, so as to form a very complete 

 defensive armour to the body ; or irregular in their shape and 



Portion of Skin of Sole highly magnified. 



separately imbedded in the skin (jplacoid), as in the sharks and 

 rays of the present day. 



The scales of almost any fish afford admirable subjects for 

 microscopic observation, but more particularly those of the 

 ctenoid kind, which exhibit a brilliancy of reflected light, and a 



