288 THE ECHINO1DEA 



valves attached to the end of a flexible stem. The valves open 

 and shut like a bird's beak, or like the avicularia of Bryozoa. 

 They have been seen to seize particles of the excreta of the urchin 

 and pass them on from one pedicellaria to another, until they fall 

 over the margin of the test. Their main function, however, appears 

 to be defensive. When a starfish attacks a sea-urchin, the latter 

 bends down its spines and thus exposes its pedicellariae ; these 

 seize hold of the tube-feet of the starfish, which their bites appear 

 to hurt. The pedicellariae, however, are always torn away, as they 

 cannot relax their hold ; and thus, if an urchin is attacked by 

 a series of starfish, it is in time rendered defenceless (Prouho, 70). 

 The sphaeridia are also modified spines ; they are globular in 

 form, and lie in pits around the peristome (Fig. V.). 



The ambulacral plates (Fig. II. 1) are ornamented by tubercles 

 and granules like those of the interambulacral plates, though 



smaller; but the plates differ 



~:vS" ----- = .-__ -- -^ in structure. The interam- 



bulacrals are solid, whereas 

 the ambulacral plates are 

 pierced by small holes, 

 through which pass the 

 suckers or podia of the am- 

 .. ., bulacra. The pores occur 



Transverse section through peristoinial plates of . L . 



Clypeaster showing a globular sphaeridium in its pit. Ill pairs, and each pair IS 



surrounded by a raised 



ring forming an epipodium. There is one epipodium to each 

 primary ambulacral plate ; but, except at the summit of the am- 

 bulacrum, the primary plates are united into compound plates, 

 each of which has as many epipodia as there are primary plates 

 in it. In one of the plates in the middle of the ambulacral series 

 there are three pairs of pores, which occur in a curved series or 

 arc. This arrangement of the pairs is due to the crowding of the 

 plates during growth ; owing to the same cause, the elementary 

 ambulacral plates no longer always extend right across the half 

 of the ambulacrum to which they belong, but are cut off from 

 the median suture by the union of adjoining plates behind them. 

 Plates thus cut off from the central suture line are known as 

 "demi-plates." In an ordinary Echinus esculentus each compound 

 ambulacral plate (Fig. XII. 4) consists of one central demi-plate 

 (d) between two primary plates (p). 



The principal remaining elements in the skeleton are those of 

 the five jaws, and of the internal processes, to which the muscles 

 that work the jaws are attached. The Dental Apparatus (Fig. VI.) 

 is a conical structure which is placed apex downwards over 

 the mouth. The axis is hollow and contains the pharynx or 

 commencement of the oesophagus. The dental apparatus consists 



