382 ZOOLOGY SECT. 



bilateral symmetry. 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. 348), 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 

 ambulacral and five inter-ambulacral areas. In the Ophiuroidea 

 and Asteroidea two of the rays constituting the bivium have 

 between them the madreportte, 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 dorsal or abactinal and ventral or 

 actinal surfaces are more or less distinctly recognisable. In the 

 Asteroidea, Ophiuroidea, and Echinoidea, the ventral 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 dorsal 

 or 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 ventral 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 dorsal surface 

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

 forms it has connected with it whorls of slender curved appen- 

 dages, 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, dorsal 

 and ventral surfaces corresponding to those of the other classes are 

 not recognisable ; but in many, as for example in Colochirus, there 

 is a marked difference between one surface which is habitually 

 directed upwards, and another which is habitually directed down- 

 wards : these dorsal and ventral surfaces in the Holothurian, it 



