486 



THE ANATOMY OF INVERTEBRATED ANIMALS. 



constituting the great mass of the corona ; and ten single 

 plates, which form a ring around its aboral or apical margin. 

 The twenty series of longitudinal plates are disposed in ten 

 double series — five ambulacral and five interambulacral — 

 alternating with one another throughout the circumference of 

 the corona. Each double series of plates presents a zigzag 

 suture in the middle line, formed by the alternating arrange- 

 ment of the triangular extremities of its component elements. 

 The sutures between the respective series of ambulacral and 

 interambulacral plates, on the other hand, are less obvious 



Fig. 141.— Diagram exhibiting the relations of the different systems of organs in an 

 Echinus.— a, mouth ; 6, teeth; c, lips: tf, alveoli ; Plaices;/, auriculae: £. re- 

 tractor, and h, protractor, muscles of lantern ; ?', madreporic canal ; A*, circu- 

 lar ambulacral vessel ; /. polian vesicle ; m, n, o, ambulacral vessel : p, pedal vesi- 

 cle ; q, q. pedicels ; r. spine ; s, tubercle to which it is articulated ; t, pedicellariae ; 

 u, anus ; v, madreporic tubercle ; x, ocular spot. 



and more straight. Each ambulacral plate is subdivided by a 

 greater or less number of sutures, which traverse it obliquely, 

 into a corresponding number of minor plates ; and these, in- 

 asmuch as they are perforated by the canals or pores, which 

 give exit to the two vessels whereby each pedicel is placed in 

 communication with its basal vesicles and with the ambula- 

 cral vessel, are called pore-plates. Throughout the greater 

 part of the length of an ambulacrum of the common Echinus 



