CHAPTER XX 



SUB-PHYLUM IV. CRANIATA 



DIVISION IT. GNATHOSTOMATA. 



SUB-DIVISION I. ANAMNIA 



Class I. PISCES 



THE class Pisces, or true Fishes, are not, as many would imagine, 

 characterised by their gills (since some Amphibia 

 retain these throughout life), but by their fins. In 

 addition to the vertical flap of skin with which we have become 

 acquainted in the case of the Cephalochorda and the Cyclostomata, 

 we find typically two pairs of lateral flaps, an anterior pair called 

 the pectoral fins, and a posterior pair known as the pelvic fins 

 (Figs. 224 and 22.5). Both from a study of their development and 

 their condition in the oldest fishes, it is believed that the paired fins 

 are derived from the division of two originally continuous lateral 

 flaps, of which the intermediate portions have disappeared. It is 

 possible even probable that the wall of the atrial cavity of Amphi- 

 oxus is homologous with this lateral fold : if this be so we may 

 argue that the absence of lateral fins in the Cyclostomata is a 

 result of secondary degeneration since Amphioxus represents a 

 much more primitive level in the evolution of the Vertebrate stem 

 than Cyclostomata. 



While the possession of paired fins discriminates Pisces from 

 the lower Vertebrata, the forms of these members equally sharply 

 mark Pisces off from the class with which they are most nearly 

 allied, namely Amphibia. In all Pisces the limb or fin is a blade- 

 like organ which never exhibits the slightest resemblance to the 

 typical form familiar to all in the human limb, but Amphibia 

 have as representatives of the paired fins limbs in which the 

 plan of the human arm and leg can be at once recognised. The 

 blade-like type of fin is known as the ichthyopterygium 



