416 ZOOLOGY sect. 



plane is, as already explained (p. 377), indicative of the bilateral 

 form of .sj'mnietry. 



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 

 amhulacral and five iyiter-amhulacral 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 dor- 

 sal 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 correspond- 

 ing to those of the other classes are not distinguishable ; but in 

 many, as for example in Colochirus, there is a marked difference be- 

 tween one surface — the dorsal, which is habitually directed upwards, 

 and another — the ventral, which is habitually directed downwards. 



In considering the general external form in the various classes 

 of Echinoderms, we have to take into account the arrangement of 

 the tube-feet — the organs of locomotion — as these have important 

 relations to the other parts and to the whole plan of organisation 

 of the animal. These organs, as previously explained, are tubular 

 appendages with highly elastic and contractile muscular walls, 

 capable of being stretched out so as to extend a long way from the 

 surface of the body. In the majority of cases the tube-foot has at 



