RADIAL STRUCTURE. 117 



some forms (certain Echinoids) there is a tendency to bilateral 

 symmetry. Speaking generally the body may be described as 

 spherical or discoidal in form, with the mouth in the centre of 

 the lower * surface and the anus at or near the centre of the 

 upper surface. An oral and aboral surface or pole may thus be 

 distinguished. The anus varies more in position than does the 

 mouth. The mouth is nearly always in the centre of the oral 

 surface or at the oral pole ; in a few Holothurians, some Echinoids 

 and in Actinometra among Crinoids it is slightly shifted from 

 this position. The anus on the other hand is central only in 

 Holothurians, and even in some of these it is slightly displaced ; 

 in Asteroids and regular Echinoids it is very near the centre of 

 the aboral surface, but always slightly excentric ; in irregular 

 Echinoids it is at some distance from the central point and 

 sometimes on the oral surface ; and in Crinoids it is always on 

 the oral surface. In a few Asteroids (Astropectinidae, etc.) and 

 all Ophiuroids the anus is absent in the adult. 



The radial structure is indicated externally by the rows of tube- 

 feet (p. 129) which extend outwards from the mouth towards the 

 aboral pole. The surface of the body is thus marked into radii 

 along which the tube-feet are arranged, and into interradii 

 the portions between the tube-feet rows. In Asteroids, Ophiu- 

 roids, and Crinoids the radial portions of the disc are prolonged 

 into processes, which constitute the arms ; in Holothurians and 

 Echinoids the radii are not so prolonged and there are no arms. 

 The rows of tube-feet never extend quite to the aboral pole ; 

 it is therefore possible to distinguish that portion of the surface 

 of the body from which tube-feet project, as ambulacra! or actinal, 

 from the antambulacral or abambulacral or abactinal surface 

 which is without tube-feet. In the brachiate Echiiioderms, that 

 is in Asteroidea, Ophiuroidea and Crinoidea, these two regions 

 or surfaces are about equal in extent, the whole of the oral 

 surface being ambulacral, and the whole of the aboral surface 

 antambulacral. In such cases it is customary to call the 

 lower or ambulacral surface ventral, and the upper or antam- 

 bulacral surface dorsal. But it is better not to use the terms 

 dorsal or ventral in adult Echinoderm morphology without 

 prefixing the word adult, because dorsal and ventral are used 



* In Crinoids and their allies the oral surface is turned upwards in the 

 natural position of the animal. 



