PARTS OF ANIMALS, IV. xiii. 



The peculiarity whicli marks oft' fishes from the GiUiu 

 other blooded animals is the possession of gills. It 

 has been explained in the treatise on Respiration ° why 

 they have them. All fishes have coverings over their 

 gills, except the Selachia, none of which have them. 

 This is because their bones are cartilaginous, whereas 

 other fishes' bones are of fish-spine, and this is the 

 substance out of which the coverings are made. And 

 again, the Selachia move sluggishly owing to their 

 lack of fish-spine — and of sinews — while the spinous 

 fishes move quickly, and the movement of the cover- 

 ing must be a quick one, for gills are a medium for 

 expiration of a sort. On this account in the selachian 

 group of fishes the passages of the gills can close up 

 by themselves, and no covering is needed to make sure 

 they close quickly. 



Now some fish have many gills, some have few ; 

 some have double ones, some single. The last one 

 is nearly always a single one. (For precise details 

 consult the Anatomical treatises and the Researches 

 upon Animals.^) The number of gills depends upon 

 the amount of heat in the heart. The more heat an 

 animal has, the quicker and stronger must be the 

 movement of its gills ; and if the gills are numerous 

 and double they are better adapted for this than if 

 they are few^ and single. And on this account, some 

 fishes (e.g. the eels and the serpentine fishes) which 

 need but little cooling, as is shown by their having 

 only a few weakish gills, can live a long time out of 

 water. 



Fish differ also with regard to the mouth. Some Mouth, 

 have their mouth right at the tip, straight in front ; 



• At 476 a 1 ff., 480 b 13 flf. 

 ^ At 504 b 28 ff. 



423 



