CARTILAGINOUS FISHES. 257 



Home is of opinion that the water is, in these cases, 

 both received and discharged by these lateral openings ; 

 v and the vulgar opinion is, that it is received by these 

 and discharged by the nostril. The last opinion can- 

 not be true, as the nostril is not connected with the 

 organs of respiration at all ; and even that of Sir Eve- 

 rard is suspicious, as it involves not only a violation of 

 analogy, but a want of skill not to be found in any 

 other production of nature. If the gill-openings are 

 adapted for the ingress of water, they would have this 

 property at all times, and the thoroughfare by the 

 mouth would be useless. But in a fish which, like the 

 shark, swims with great velocity, the entrance of the 

 water by the lateral openings would be accomplishing 

 a purpose by the most difficult means. Those openings 

 are behind rather than before the gill-cells, so that the 

 mechanical action of the animal through the water 

 would prevent that fluid from entering by those aper- 

 tures, though it would facilitate it in getting out. So 

 obvious is this, upon the very simplest principles of 

 motion in a fluid, that one can hardly imagine the oppo- 

 site possible ; and so much is it the case in osseous 

 fishes, that if they be drawn backward with rapidity, 

 or held by the tail in a current, they are speedily 

 drowned, strangulated, because the gills will not act. 

 When fishes of this kind are swimming, they must 

 therefore take in water by the mouth, and pass it out 

 by the gill openings, just in the same way as other 

 fishes. Still, the respiration while sucking is a diffi- 

 culty, though the difficulty does not consist, as Sir 

 Everard thinks, in how the water gets out, but in how 

 it gets in ; and that difficulty is not cleared up by his 

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