82 ZOOLOGY. 



(sharks, skates, sturgeons, lampreys) it is cartilaginous. 

 The skin is usually covered with thm translucent 

 scales; but there are fishes with a smooth skm 

 (lampreys), with prickle-like dermal ossifications 



Fig. 53. — The Perch (Percajluviatiiis). 



(sharks), and with rhomboidal bony plates (sturgeons). 

 As Fishes are without exception aquatic, there is no 

 need for me to treat more specially of them here. 



Second Sub-Kingdom : ARTHROPODA 



(JOINTED-LIMBED AnIMALS). 



The body of an Arthropod is bilaterally sym- 

 metrical (cp. p. 16), and consists of a number of 

 joints (Fig. 55), not equally numerous in all members 

 of the sub-kingdom. These joints or segments lie one 

 behind the other, and are at first alike; but as 

 in the course of further development they become 

 adapted to various functions, the difference between 

 them becomes greater. Compare the Wood-borer 

 represented in Fig. Si, 2, with the young form of the 



