ECHINOIDEA 647 



madreporite, however, the sinus is free and enlarged and forms an 

 "ampulla" into which the stone canal opens. 



The oral ring of the lacunar system lies below the water vascular 

 ring. The features of this system have been described on p. 629. The 

 gonads are five large masses hanging into the perivisceral cavity from 

 the region of the genital plates. The rachis is degenerate in the adult. 



Echinus is an example of the regular sea urchins (order Endocyclica). 

 The class contains two other orders, Clypeastroida and Spatangoida, 

 known collectively as the irregular urchins (Exocyclica), in which the 

 anus, with its periproct, is displaced from the apical position which it 

 occupies in the regular forms, and lies in an interradius, known as the 

 posterior interradius, so that the body, which is considerably or very 

 much flattened, has a marked bilateral symmetry. The madreporite 

 remains in position and extends over the region vacated by the peri- 

 proct. In most of the irregular urchins (though not in certain primi- 

 tive forms known as Holectypoida or Protoclypeastroida) the aboral 

 parts of the ambulacra are expanded to an oval shape (petaloid) and 

 bear flattened, respiratory tube feet. These peculiarities are associated 

 with the habit, possessed by typical members of both orders, of living 

 partly or wholly buried in sand (see below). 



Order ENDOCYCLICA 



Echinoidea in which the mouth is central, the anus remains within 

 the apical area, and the ambulacra are not petaloid. 



Echinus (Figs. 447-453). A typical example, described above. 



Order CLYPEASTROIDA 



Echinoidea in which the mouth is central and furnished with a 

 lantern, the anus outside the apical area, the dorsal parts of the am- 

 bulacra nearly always petaloid, and the body usually much flattened. 



The members of this order live at or near the surface of the sand, 

 and walk by means of the tube feet, which are ver)/- numerous. They 

 extract food from the sand, which they shovel into the mouth by 

 means of the teeth. 



Clypeaster. A typical member of the group, of large size, wide- 

 spread in tropical waters. 



Echinocyamus. Small, oval, and not extremely flattened. E.pusillus 

 is a British species. 



Order SPATANGOIDA 



Echinoidea in which the anus and often also the mouth are excentric, 

 the lantern has disappeared, the dorsal parts of the ambulacra are 

 petaloid, and the body cushion-shaped or heart-shaped. 



