CHAPTER X 



FISHES 



Elasmobranchii. Sharks, Dogfish ; Skates, Rays ; (Holocephali). 

 Teleostomi. 



Crossopterygii. Polypterus. 

 Actinopterygii. 



Ganoidei. Sturgeons, Lepidosteus, Amia. 

 Teleostei. Ordinary Fish. 

 Dipnoi. Lung-fish. 

 Appendix. 



1. Cephalochorda. Amphioxus. 



2. Cyclostomata. Lampreys {Petromyzon) ; Hagfish (Myxine), Borer 

 (Bdellostoma). 



In scientific as in everyday language there are grouped together under 

 the name Fishes those lower vertebrates that are specially adapted to 

 a swimming existence. They are linked together by a number of common 

 features. Their paired limbs are in the form of fins and they possess 

 also unpaired fins with skeletal supports. They retain throughout life 

 open gill-clefts and use them for respiration. The spiracle does not 

 become modified in relation with the sense of hearing as is the case 

 with terrestrial vertebrates. The main muscles of the body retain their 

 primitive segmentation into myotomes throughout life. 



A few different genera of fish show the remarkable structures known 

 as electric organs. These are, with one possible exception, composed of 

 modified muscles. Muscle in general may be said to be living substance, 

 specialized for the function of contraction. When it actually functions 

 the obvious thing that happens is the change of form — shortening and 

 thickening — of the muscle, but accompanying this is a much less 

 conspicuous subsidiary phenomenon namely the production of a slight 

 electric disturbance. Now in electric organs the primitively predominant 



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