DESCRIPTION 



615 



eral shape of the body. In some echinoderms this axis is very short, 

 as in the starfish, the oral and aboral surfaces are near together, and 

 the body is flat and disc-shaped; in these animals the oral surface is 



Fig. 965 Diagrams illustrating the radiate structure of echinoderms. A, star- 

 fish, oral aspect ; B, sea urchin, oral aspect ; C, sea urchin, lateral aspect ; D, holo- 

 thurian, lateral aspect (Boas). 1, radius; 2, interradius ; 3, mouth; 4, anus. 



the under one, on which the animal moves about, and the main body 

 axis is perpendicular to the ground. In others the main axis is 

 long and the body is 

 consequently long and 

 worm-like, as in holo- 

 thurians; the oral sur- 

 face, with the mouth, 

 is not undermost in 

 these animals, but is at 

 what may be called its 

 anterior end, and the 

 animal lies with its 

 main axis parallel with 

 the ground. In crinoids 

 the oral surface lies 

 uppermost (Fig. 968), 

 and in many of them a 

 long stalk extends from 

 the aboral surface at- 

 taching the animal to 

 the sea bottom. 



In starfishes, brit- 

 tle-stars, and crinoids 

 the radii are greatly elongated, forming five or more large arms, which 

 radiate from a central disc (Fig. 965, A). Many echinoderms also show 

 a decided tendency towards the development of a secondary bilateral 

 symmetry and always move with a certain radius in advance (Fig. 



ig. 



showing the arrangement of certain of the organs! 

 A, a section through the disc and one arm of a star- 

 fish ; B, a section through a sea urchin (altered from 

 Boas). 1, mouth; 2, anus; 3, peristome; 4, ring 

 canal ; 5, stone canal ; 6, stomach ; 7, radial canal ; 

 8, ampulla ; 9, madreporite ; 10, ambulacral foot ; 11, 

 liver ; 12, dentary body ; 13, gill ; 14, calcareous 

 plates ; 15, papula. 



