564 THE INVERTEBRATA 



The alimentary canal (Fig. 421) is a mere bag, not protrusible 

 through the mouth, which is armed with an arrangement of spines 

 serving as teeth. The food of some species consists of animals cap- 

 tured by the arms: others shovel mud into the mouth with the ad- 

 jacent tube feet and digest the food it contains. There is no anus. 

 The madreporite, aboral in the young, becomes oral in the adult 

 because the disc, growing independently of the arms, and faster 

 aborally, comes to overhang in the interradii. In coming over, the 

 madreporite brings with it the axial sinus, stone canal, axial organ and 

 madreporic vesicle, which are all orally placed. The gonads open, not 

 directly to the exterior, but into genital bursae, of which one opens on 

 each side of the base of each arm (Fig. 4 19,^). The ectoderm lining the 

 bursae retains its cilia and causes currents which subserve respiration. 



Ophiuj'a, Ophiocoma, Ophiothrix, Amphiura. British genera, 

 separated by relatively unimportant differences, which are chiefly 

 evident in the ossicles and spines. Amphiura is hermaphrodite and 

 viviparous. 



Class ECHINOIDEA 



Globular, cushion-shaped, or discoidal Echinodermata, without 

 arms ; with small abambulacral area, in which lies the madreporite ; 

 ambulacral grooves covered; tube feet ending in suckers; numerous 

 long spines; and pedicellariae. 



The characteristic form of body of the Echinoidea is such as would 

 result if the arms of a starfish were drawn up into the body by shrink- 

 age of the aboral surface. 



We shall describe the anatomy of this group by an account of a 

 typical member of it — Echinus esculentus^ a large species common in 

 Britain. This animal (cf. Fig, 422) has the shape of a sphere with one 

 side flattened, slightly polygonal in equatorial outline. In the middle 

 of the flattened side is the mouth. Under the delicate, ciliated epi- 

 dermis an armour, the shell or corona, composed of dermal plates 

 firmly sutured together, encloses most of the body, but at the two 

 poles there are leathery areas, the peristome around the mouth, and 

 the periproct in which the anus lies excentrically. The corona (Fig. 

 423) is composed of twenty meridional rows of plates, two in each 

 radius (ambulacrum), and two in each interradius. The plates of the 

 ambulacra are distinguished by the presence on them of the pores 

 for the tube feet. These pores are in pairs, since each ampulla com- 

 municates with its tube feet by two canals. Thus water can circulate 

 in and out of the tube feet and respiration is facilitated. At the aboral 

 pole each radius ends in a single ocular plate, which bears the opening 

 of the terminal tentacle, and each interradius in a genital plate which 



