CHAPTER IX 



CHARACTERS UF THE CLASS PISCES — THE J)()(;ITSH 



The class Fisces (sec p. 403) includes a number of aiiuatic 

 A'ertcbrates which present a considerable amount of differ- 

 ence in form and structure. Excluding, however, one small 

 group, the " Mud-fishes " or Dipnoi — which differ so much 

 from other fishes that they arc sometimes placed in a 

 separate class, intermediate between Fishes and Amphibians 

 — the Pisces are distinguished from the Amphibia, as a 

 whole, by certain constant characteristics, of which the 

 following are the chief. 



The organs of respiration and locomotion are adapted for 

 life in the water. 'I'he former consist, as in the tadpole 

 (p. 207), of a series of vascular processes, the ,i,77/.v, attached 

 to the arches separating the gill-clefts and ])cr',isting 

 throughout life; and lungs arc never developed. The 

 pectoral and pelvic limbs have the form of paddle-like 7?//^', 

 which, like the median fins (p. 406), are supported by skele- 

 tal fin-rays ; the median fin is usually subdivided into 

 separate dorsal, ventral, and caudal portions. In addition 

 to the endoskeleton, there is usuall\- an exoskelcton, de- 

 \ eloped in the derm and consisting of scales ; and peculiar 

 intent! iiieiita7'y sense-organs, supplied b\' special nerves not 

 re[)rcsented in terrestrial ^'ertebrates, are always present. 



