HOLOTHUROIDEA. 265 



the embryo passes at once into the stage of the barrel-shaped 

 larva. It is said that the eggs of Cucumaria kirchsbergii are 

 already fertilized when they leave the body of the mother. 



With regard to affinities we think that there can be no doubt 

 that the Holothurians must be placed apart from the three 

 preceding classes of Echinodermata. They differ from these 

 in a number of anatomical features which we cannot but regard 

 as of fundamental importance. For instance, in the embryo, 

 the inconspicuousness of the anterior coelom ; in the adult, the 

 absence of radial repetition of the gonad, and the absence of 

 the axial sinus and axial organ, and the absence at all times 

 of life of the plates of the apical system. The absence of 

 pentamerous structure in the alimentary canal cannot be re- 

 garded as so important, for the same negative feature is charac- 

 teristic of Echinoidea. Neither can the fact that the blastopore 

 gives rise to the permanent anus, or that the larval mouth 

 and anus both persist into the adult, be regarded as of funda- 

 mental importance having regard to the varied behaviour of 

 these structures in other animals. But although it is undoubted 

 that the Holothurians must be placed apart from Asteroids and 

 Echinoids, still it cannot be said that they approximate to the 

 Crinoids, for those very features which separate them from the 

 first, separate them also from the last. The anterior coelom, 

 the radial repetition of the gonads, the axial organ and the 

 apical plates are all present and well-developed in Crinoids, 

 though the anterior coelom disappears in the adult. Indeed we 

 may go further and say that Holothurians stand further from 

 Asteroids and Echinoids than do the Crinoids, for whereas 

 Asteroids and Echinoids agree with Crinoids in the above- 

 named anatomical characters, the only point of importance 

 which they have in common with Holothurians and which is not 

 also found in Crinoids is the form of the free larva. For these 

 reasons we hold that out of the primeval matrix of the Echino- 

 dermata three main groups have emerged and persisted to the 

 present day ; these are (1) the Asteroids, Ophiuroids and 

 Echinoids, (2) the Holothuroids and (3) the Crinoids. 



Order 1. ACTINOPODA. 



All external appendages of the water-vascular system arise from the 

 radial canals, and have the form of tentacles round the mouth and of 

 ambulacral feet and papillae on the body. Tentacles are always present, 

 but feet and papillae may be absent. 



