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 .difference in form and structure, and which 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. The former consist, as in later 

 stages of the tadpole (p. 207), of a series of vascular 

 internal gills, attached to the septa separating the gill- 

 clefts, and they persist throughout life : only in a very 

 few instances (viz., in the small sub-class of the 

 Dipnoi or " Mud-fishes ") are lungs and internal nostrils 

 also present. The pectoral and pelvic limbs have the 

 form of paddle-like fins, which, like the median fins 

 (p. 421), are supported by skeletal fin-rays : the. median 

 fin is usually subdivided into separate dorsal, ventral, 

 and caudal portions. In addition to the endoskeleton, 

 there is usually an exoskeleton, developed in the derm 

 and consisting of scales ; and peculiar integumentary 

 sense-organs, occurring also in aquatic and larval 

 Amphibians, and supplied by special nerves not repre- 

 sented in terrestrial Vertebrates, are always present. 

 The cerebellum is relatively large ; a tympanic cavity 



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