CHAPTER X 



CHARACTERS OF THE CLASS PISCES THE DOGFISH 



The class Pisces (see p. 418) includes a number of aquatic 

 Vertebrates which present a considerable amount of differ- 

 ence in form and structure, and which are distinguished 

 from the Amphibia, as a whole, by certain constant charac- 

 teristics, of which the following are the chief. 



The organs of respiration and locomotion are adapted for 

 life in the water. The former consist, as in the tadpole 

 (p. 207), of a series of vascular processes, the gifts, attached 

 to the septa separating the gill-clefts and persisting 

 throughout life : lungs, and internal nostrils, are also present 

 in a very few instances (viz. in the small sub-class . consti- 

 tuting the " Mud-fishes " or Dipnoi}. The pectoral and 

 pelvic limbs have the form of paddle-like fins, which, like 

 the median fins (p. 421), are supported by skeletsiljin-rays : 

 the median fin is usually subdivided into separate dorsal, 

 ventral, and caudal portions. In addition to the endo- 

 skeleton, there is usually an exoskeleton, developed in the 

 derm and consisting of scales ; and peculiar jutf. 



sense-organs \ like those of aquatic and larval Amphibians, 

 and supplied by special nerves not represented in terrestrial 

 Vertebrates, are always present;. The cerebellum is rela- 



