326 THE ECHINOIDEA 



the abactinal poles of Asteroids and Crinoids, on which these plates 

 develop, have nothing to do with one another. MacBride recognises 

 the influence of fixation on the anatomy of Starfish and Echinoids, 

 but maintains that such fixation was by the actinal surface in 

 these two classes, whereas in Crinoids it was by the abactinal 

 surface. (See, however, Chapter VIII. p. 14.) 



The question as to which of the groups of organs first acquired 

 the radial character is of great importance in connection with the 

 origin of the Echinoderma. The Sarasins, who made a detailed 

 study of the Echinothuridae, were much impressed with the 

 importance of the radial muscles, and suggested that it was the 

 muscles that first became pentamerous. There are many striking 

 points of resemblance between such a form as Asthenosoma and the 

 Holothurians. The Sarasins therefore ridiculed the supposed Crinoid 

 ancestor as a " Crinoid phantom," and derived the Echinoidea from 

 the Holothurians. This argument is based on the idea that the 

 primitive characters of the Echinothuridae are due to inheritance 

 from the ancestral Echinoid. But it appears most probable that 

 the Echinothuridae arose from the Diademoid Pedina, or from 

 some close ally of that genus. The primitive characters of the 

 Echinothuridae are therefore secondarily acquired, and are not 

 original. The immediate ancestor of the family lived in the 

 Jurassic, and not in the Palaeozoic seas. To accept this con- 

 clusion means to abandon the derivation of the Echinoids from the 

 Holothurians. 



Leuckart, in 1848, separated the Echinoderma into the three 

 groups of the Pelmatozoa, Echinozoa, and Scytodermata, and this 

 classification is still generally used in practice. In the two latest 

 arrangements of the Echinoderma, those of Bell and Haeckel, the 

 Echinoids are still grouped with the Stelleroids. They undoubtedly 

 agree in several important characters, the members of both classes 

 having the gonads pentamerous, the oral surface kept downward, 

 and power of locomotion. 



The class Stelleroidea is older than that of the Echinoidea, but 

 we know of no member of the former that can be regarded as the 

 ancestor of the latter. 



The evidence in favour of the origin of Echinoids from Cystids 

 or allied forms is more weighty. Neumayr advocated this view 

 (64, 65), and it has recently received fresh support from Haeckel 

 (36 on p. 213). Neumayr included Echinocystis in the Cystidea. 

 The genus is, however, here included among the Echinoids. The 

 uncertainty as to its position shows that there is an approximation 

 between the two classes. We are, therefore, forced to the position 

 that one group l of primitive Pelmatozoa diverged from the mam 

 stem and approximated to the Echinoids ; and that it was succeeded 



1 Separated in Chapter XII. as a Class Edrioasteroidea. 



