170 INVERTEBRATE ZOOLOGY 



with other ambulacral plates. The notches in adjacent plates 

 coincide, so the opening for each ambulacrum is between two 

 plates. The ambulacral pores thus formed are in parallel rows. 

 Distally, each double row of ambulacral plates terminates in a 

 single ocular plate. This is so named because of the sense organ 

 which it bears. The rigid body of a dried or preserved specimen 

 gives little idea of the powers of movement possessed by the 

 arms of the living starfish. The plates are articulated and their 

 movements are controlled by body muscles in a manner which 

 permits of considerable flexibility and freedom of movement 

 in the arms. Laterally, yet on the oral surface, the ambulacral 

 plates are bordered by a row of interambulacral plates which 

 usually bear movable spines. A series of less regularly arranged 

 adambulacral plates edges each row of interambulacrals. 



The aboral surface of each arm is made up of a series of plates 

 of considerably variable arrangement in different members of the 

 group. The disc of the Asteroidea lacks the regularity in 

 arrangement of its plates so characteristic of some other classes of 

 echinoderms. The madreporite is the only conspicuous plate 

 which is constant in position, and even this is variable in some 

 species, for in these more than one madreporite occurs. Both the 

 disc and the rays are typically covered with scattered spines. 

 The areas between these spines are ciliated and bear numerous, 

 small, hollow filaments, the branchiae or gills, which are direct 

 continuations of the coelom. Through the thin walls of these 

 gills the body fluids are able to carry on the respiratory process. 



Pedicellariae. — Surrounding the body spines and scattered 

 over the general surface of the body, there are frequently minute 

 pincher-like organs called pedicellariae. Each of these is a small 

 calcareous organ at the end of a strongly muscular stalk. In 

 various species, these differ in form though two types are 

 commonly found. In one type, the two jaws of the pinchers are 

 articulated at their bases with a separate small calcareous body, 

 while in another type the two jaws cross each other at their 

 bases and continue beyond the crossing in handle-like processes 

 where the muscles for operating the jaws are attached. Pedicel- 

 lariae serve to remove small foreign bodies from the skin of the 

 starfish and probably also serve as protection against small 

 organisms which might attack the body of the starfish. 



Plates of the Disc. — The five genital plates, which are located 

 interradially on the dorsal surface of the disc, are perforated by 



