ix PHYLUM ECH1NODERMATA 407 



merit serves to hide a bilateral symmetry which may be more 

 primitive. This is best marked in the larva, which has 

 pronounced bilateral instead of radial symmetry, but is quite 

 recognisable in the adult. In all Echinoderms there is, passing 

 through the primary axis, a plane the median plane along 

 which, and along which alone, the body is capable of being divided 

 into two equal or, to speak more correctly, approximately equal- 

 right and left halves. The existence of such a single median 

 plane is, as already explained (p. 43), indicative of the bilateral 

 form of symmetry. 



The body is most usually five-rayed (Ophiuroidea, most Aste- 

 roidea, Crinoidea), cylindrical (most Holothuroidea), or globular 

 (most Echinoidea), the surface in the two last cases being marked 

 by five bands or zones of tube-feet, which divide it into five 

 ambulacra! and five inter -ambulacral areas. In the Ophiuroidea 

 and Asteroidea two of the rays constituting the bivium have 

 between them the madreporite, marking the position of the 

 madreporic canal of the ambulacral system ; the remaining three 

 rays form the trivium. The median plane passes through the 

 madreporite, and thus midway between the two rays of the 

 bivium, and bisects longitudinally the middle ray of the trivium. 

 A corresponding disposition of the parts is traceable also, as will 

 be subsequently shown, in the cylindrical and globular Echinoderms. 

 In all the Echinodermata aboral or abactinal and oral or actinal 

 surfaces are more or less distinctly recognisable. In the 

 Asteroidea, Ophiuroidea, and Echinoidea, the actinal surface is 

 that in the middle of which the mouth is situated, and which is, 

 in the natural position of the animal, directed downwards or 

 towards the surface to which it is clinging. The opposite abactinal 

 surface is, in the majority of the Asteroidea and Echinoidea, 

 marked by the presence of the anal aperture : in the Ophiuroidea 

 and some Asteroidea the anus is absent ; in some Echinoidea it is 

 situated on the border between the two surfaces, or even on the 

 - oral surface. In the Crinoidea the oral surface, which is habitually 

 directed upwards in the natural position of the animal, bears both 

 mouth and anus, the former central, the latter eccentric and inter- 

 radial. In the fixed Crinoids the abactinal or aboral surface has 

 attached to its centre the distal end of the stalk ; in the free forms it 

 has connected with it whorls of slender curved appendages, the 

 dorsal cirri, by means of which temporary attachment is effected. 

 In the Holothurians, owing to the elongation of the body in the 

 direction of the line joining mouth and anus, oral and aboral surfaces 

 corresponding to those of the other classes are not distinguishable ; 

 but in many, as for example in Colochirus, there is a marked 

 difference between one surface the dorsal, which is habitually 

 directed upwards, and another the ventral, which is habitually 

 directed downwards. 



