IV. 13- 137 



use the marginal parts of their flattened bodies in place of fins 

 for swimming.^'^ 



In the Torpedo and the Fishing-frog the breadth and flatness 

 of the anterior part of the body is not so great as to render 

 locomotion by fins impossible. It necessitates, however, the dis- 

 placement of the upper pair to a point further back, and the 

 advancement of the under pair to the head. At the same time 

 to compensate for this advancement these lower fins are reduced 

 in size so as to be smaller than the upper ones.'* In the Torpedo 

 the two upper fins are placed on the tail, and the fish uses the 

 broad expansion of its body to supply their place, each lateral 

 half of its circumference serving the office of a fin.*^ 



The head, with its several parts, as also the organs of sense, 

 have already come under consideration.^" 



There is one peculiarity which distinguishes fishes from all 

 other sanguineous animals, namely, the possession of gills. Why 

 they have these organs has been explained in the treatise on 

 Respiration.'-^' These gills are in most fishes covered by opercula, 

 but in the Selachia, owing to the skeleton being cartilaginous, 

 there are no such coverings. For an operculum requires fish-bone 

 for its formation, and in other fishes the skeleton is made of this 

 substance, whereas in the Selachia it is invariably formed of car- 

 tilage. Again, while the motions of bony fishes are rapid, those 

 of the Selachia are but sluggish, owing to the absence of bone 

 and of sinew. But an operculum requires rapidity of motion, 

 seeing that the office of the gills is to minister as it were to expira- 

 tion.'^^ For this reason in Selachia the branchial orifices them- 

 selves effect their own closure, and thus there is no need for an 

 operculum to ensure its taking place with due rapidity .^^ In some 

 fishes the gills are numerous, in others few in number ; in some 

 again they are double, in others single. The last gill in most cases 

 is single.^* For a detailed account of all this, reference must be 

 made to that part of the treatises on Anatomy which relates to 

 fishes, and to the book of Researches concerning Animals.^^ 



It is the abundance or the deficiency of the cardiac heat which 

 determines the numerical abundance or deficiency of the gills. 

 For, the greater an animal's heat, the more rapid and the more 

 forcible does it require the branchial movement to be ;2^ and 

 numerous and double gills act with more force and rapidity than 

 few and single ones, Thus too it is that some fishes that have 

 696b. 



