CHAPTER IX 



CHARACTERS OF THE CLASS PISCES — THE DOGFISH 



The class Pisces (see p. 403) includes a number of aquatic' 

 Vertebrates which present a considerable amount of differ- 

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

 group, the " Mud-fishes " or Dipnoi— \\\\vi}ix differ so much 

 from other fishes that they are 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. The former consist, as in the tadpole 

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

 to the arches separating the gill-clefts and persisting 

 throughout life ; and lungs are never developed. The 

 pectoral and pelvic limbs have the form of paddle-like yf«j, 

 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 usually an exoskeleton, de- 

 veloped in the derm and consisting of scales ; and peculiar 

 integumentary sense-organs, supplied by special nerves not 

 represented in terrestrial Vertebrates, are always present. 



