184 



SUBKINGDOM VERTBBEATA. 



Mg. SIS. 



CLASS V. PISCES. 



General Characteristics. — Fishes rank lowest among 

 Tertebrates. Their bodies occupy a horizontal position, with 



no power even of elevating 

 the head independently. 

 Their senses are blunted. 

 Their eyes are large, but 

 generally have no motion. 

 The external ears are want- 

 ing. They progress mainly 

 by horizontal movements of 

 the vertical tail, in marked 

 contrast with the vertical 

 movements of the horizon- 

 tal tail of the mammals in- 

 habiting the water. The 

 two pairs of fins underneath, 

 corresponding to the four 

 limbs of quadrupeds, as well 

 as the fins upon the back, 

 serve as balancers. Most 

 fishes have a swimming-blad- 

 der — a sort of rudimentary 

 lung — whose use is doubt- 

 ful, though it is thought to 

 be serviceable in changing 

 their specific gravity. They 

 breathe by means of gills. 

 These are delicate fringes or 

 ciRouLATOBT SYSTEM OF FisH.-a. Bran- laminae on each side of the 

 chicu artery ; b. Arterial bulb ; c. ven- head, generally protected by 



tride of heart; d. Auricle : e. Venous i t i n j ii, -77 



slr^us;!. Vena pm-tm, etc. ;e..I«teeane; » hony lid called the gill- 



h. Tena cma ; i. Gill vessels ; k. Dm-sal cOver. The Water, usually 



artery; I. Kidneys ;m.A,yrta. ^^^^^ j^^^^ ^^^ ^^^^^ .^ ^ 



vray similar to the act of swallowing, bathes the gills, and is then 



