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mal, the head of which is so intimately connect- 

 ed with the trunk, can be said to possess such 

 an organ. In scaly fishes they are generally 

 covered by two or three osseous plates, called 

 an operculum, and in others, as in eels, by a 

 simple membrane. In some fishes the gills are, 

 in this manner, entirely concealed, in others they 

 are but partially covered, and again in some they 

 are freely exposed. The gills generally contain, 

 on each side, four cartilaginous or osseous cir- 

 cles attached to small bones united to the pa- 

 late to each of which is fixed a double row 

 of finger-like processes, or rather penniform la- 

 minse, of a red colour, and consisting of termina- 

 tions of the large vessels already mentioned. 

 Here the blood loses its grosser particles, while 

 the oxygen, which has been spent in the course of 

 its circulation through the body, is here supplied 

 from the water, in which the gills freely play. 

 The water passes in at the animal's mouth, which 

 contains several large openings communicating 

 with the gills, over which it spreads, and, when the 

 animal shuts its mouth, escapes at the opercula 

 or gill covers. The water acts not only chemi- 

 cally, but also mechanically upon the gills by 

 separating their laminae to facilitate its access. 

 Without this wise provision, fishes would die as 

 speedily in water as they do in air, as may easily 

 be seen by compressing their gills, and thus pre- 

 venting the necessary access of the oxygenating 

 medium ; while on the other hand, the life of fishes 

 be prolonged in air by artificially keeping the 



