118 PHYLUM ECHINODERMATA. 



in an entirely different sense in describing the larvae and in 

 adult Holothurians, and cannot be used satisfactorily in Echin- 

 oids at all. In Holothurians and Echinoids the tube-feet extend 

 almost to the aboral pole, and the antambulacral surface of the 

 body is restricted to the very small region round the anus. 

 Moreover in Holothurians, in which the body is cylindrical and 

 lies with its whole length applied to the substratum, it is usual 

 to call one surface of the cylinder, viz. that on which the single 

 genital opening and madreporite are placed, the dorsal, and the 

 other side the ventral. In many Holothurians the animal always- 

 lies with the three radii (trivium) of the so-called ventral surface 

 directed towards the substratum and uses the tube-feet of these 

 radii for adhesion, the tube-feet of the two dorsal radii (bivium) 

 being without suckers and probably used for respiration and 

 sensation only. In pentameral Echinoderms the words bivium 

 and trivium are frequently used to designate the two groups 

 into which the five radii may be divided. These words are 

 used however in a somewhat vague sense and it must not be 

 supposed that the arms of the bivium and trivium of one class 

 are necessarily the same as the bivial and trivial arms of another 

 class. The water-pore or madreporite is generally single and 

 always interradial in position. It is generally abactinal, but 

 in Crinoids and Ophiuroids it is on the ambulacral surface. In 

 Holothurians the madreporitic interradius occupies the middle 

 of the so-called dorsal surface. 



In Ophiuroids as in Asteroids the water-pore is generally single, 

 but in Crinoids it is always multiple, there being one (Rhizo- 

 crinus) or more than one in each interradius. The number of 

 water-pores (madreporites *) is however subject to variation in 

 all classes of Echinoderms except Echinoids, in which normally 

 there is never more than one. 



In Echinoids, Holothuroids, Neocrinoids and Blastoids the 

 number of radii is constantly five except in abnormal individuals. f 

 In Asteroids, Ophiuroids, Crinoids and Cystids this number may 



* For complete explanation of the terms water-pore and madreporite 

 see below, p. 127. It may be mentioned here that the numerous perfora- 

 tions in the madreporitic plate of Echinoids and Asteroids represent one 

 water-pore only. 



f See Bateson, Materials for the Study of Variation, London, 1894, 

 p. 432 et seq. In Echinoids 4- and 6-rayed abnormalities are not 

 uncommon, and in Holothurians Ludwig found half a dozen 6-rayed 

 individuals in 150 specimens of Cucumaria planci. 



