SHABKa AND MATS. 143 



which, m short, are allied to the Batrachians, and through 

 them with the reptiles, birds, and mammals. All the fishes 

 agree in having a true skull, to which is attached a mova- 

 ble lower jaw. The brain is well developed and the blood 

 is red. Fishes breathe by gills, which form four arches on 



Dorsal fin. 



Caudal. 



Anal. Ventral. Pectoral. 

 Fig. 184.— The Mud-Minnow, with the names of the fins. 



each side of the throat. The body is usually scaled. They 

 are mostly oviparous; some bring forth their young alive. 



Sub-Classes op Fishes. 



1. Skeleton cartilaginous; 5-7 pairs of 



gill-openings ElusmobrancMi: Sliarks, Rays. 



2. Skeleton cartilaginous or bon}'; 



scales often square, enamelled. . <?a;wMtejV Sturgeon, Garpike. 



3. Skeleton bony, of numerous sep- 



arate bones; 4 pairs of gills Teleoslei: Cod, Perch, etc. 



Sub-Class I. — Elasmobranchii {Selachians, or Sharks 

 and Rays). 



These are called Ehismobranchs from the strap-like 

 gill-openings {elasma, strap, and lirancliia, gill). The 

 sharks, though fish-like, are very different from ordi- 

 nary bony fish. Their skeleton or skull is so soft that it 

 can be cut with a knife, while the tail is one-sided, the ver- 

 tebral column ending in the larger up]ier lobe. They also 

 have from five to seven gill-openings orslits, whereas the cod 

 or perch has but one. The skin is either smooth, or with 

 minute scales, forming shagreen. Both jaws are armed 

 with numerous sharp, flattened teeth, arranged in rows and 



