204 ECHmODERMATA. 



which are formed by the inner layer. Between two 



of the arms lies a conspicuous calcareous tubercle 



or madreporic plate, — a porous sieve through which 



fluid enters and leaves the water-vascular system. 



The two rays enclosing this madreporic plate are 



called the bivium ; the other three form the trivium. 



The Calcareous Skeleton is intricate. We shall mention 



only the more important components,— the plates forming 



the ventral groove of each arm, the small scattered plates, 



the short spines and the pedicellarise. 



(a) In association with the inner mesodermic layer of the 

 integument, there is developed on the ventral surface 

 of each arm a double series of sloping plates. These 

 two series meet dorsally, like rafters, in the middle 

 line of the arm, forming an elongated shed. The 

 rafter-like plates are called ambulacral ossicles; the 

 groove which they bound lodges the nerve-cord, the 

 blood-vessel, the water-vessel, and the tube-feet of 

 each arm. 

 {b) In association with the outer mesodermic layer of the 

 integument numerous smaller plates are developed, 

 e.g., the adambulacrals which articulate with the 

 outer lower ends of the ambulacrals. The dorsal 

 surface bears a network of little ossicles, and many of 

 these bear spines. Peculiarly modified spines, known 

 as pedicellarice, look like snapping scissor blades 

 mounted on a single soft handle. " They are used 

 to take hold of objects such as Algae, until the tube- 

 feet can be applied." 

 Muscular System. — A starfish is not very muscular, but 

 it often bends its arms upwards, and may sometimes be 

 seen tightly embracing an oyster. Other muscles affect the 

 size of the ventral grooves, and muscular elements also 

 occur on the protrusible part of the stomach, and in 

 connection with the water-vascular system. 



Nervous System. — Underneath the ciliated ectoderm lies a 

 network of nerve fibrils, with some gangUonic cells. But 

 these diffuse elements are concentrated in a pentagon around 

 the mouth, and a nerve along each arm. The system is not 

 separable from the skin. 



Sense-Organs.—K red " eye," sensitive to light, lies at the 



