152 



ARISTOTLE'S ANHOMCEOMERIA 



both sides of the head.* In a comparatively few fishes, the 

 last gills are single, but he states incorrectly that they are 

 single in all or most fishes.! Again, many gills which are 

 known to be double are said by him to be single. This may 

 be explained, in some cases, on the assumption that he 

 included among single gills those which, like the gills of the 

 eel and the sturgeon, have two sets of leaflets joined to- 

 gether for a rather large part of their length. He does not 

 appear to have known anything of the half-gills or pseudo- 

 branchs in some bony and cartilaginous fishes. 



In H. A. ii. c. 9, s. 4, Aristotle mentions many fishes 

 and gives the numbers of gills for each. In the following 

 table, the gills of some of these fishes are compared with 

 those of the fishes which seem to furnish the best identi- 

 fication of them : — 



In addition to other peculiarities of structure, the gill- 

 processes of each pair are free in the sword-fish, so that 

 there seem to be eight gills. Aristotle evidently knew of 

 this peculiarity. 



Aristotle says that the flat, cartilaginons fishes, such as 

 the torpedo and ray, have their gills below, but the long 

 ones, such as the dogfishes, have lateral gills, and that 

 the fishing-frog has lateral gills, but these have skin-like 

 opercula and not spiny ones, like fishes which are not 

 cartilaginous.! 



The Angel-fish {Rhina sqiiatina), which was known to 



* H. A. ii. C.9, s. 4. 



f H. A. ii. c. 9, s. 4 ; P. A. iv. c. 13, 6966. 

 I H. A. ii. c. 9, s. 3. 



