EOACTINID^E. 



179 



We should expect to find, when we study the fossil Asterozoa, primitive forms 

 which would be near the stage at which the Ophiuroidea and the Asteroidea took 

 their different paths of evolution. These forms would not, as yet, have pronounced 

 distinctive habits ; that is, the Ophiuroidea would not have acquired the long thin 

 arms peculiarly adapted for wriggling. We must find, therefore, some more 

 delicate test when dealing with the primitive forms. It seems to me that the 

 manner of growth of the disc is such a test, although it must be noted that even 

 this breaks down when applied to the most primitive forms (see above, p. 170). 

 Nevertheless, until some better means be devised this test must be followed, and, 



Text-fig. 122a. — Diagrams illustrating the increase of growth of the disc in the Asteroidea. 2. An angle 

 of the disc of one of the Hudsonasterida?. The liases of the arms are unfused, as shown by the entire 

 separation of the proximal infero-marginalia by the median egg-shaped odontophor. //. In the Mesopala?- 

 asteridae the first infero-marginalia of neighbouring arms have been brought together by the beginnings 

 of arm-fusion, and have enclosed the odontophor. III. In Promopalceaster belhdvs the fusion of the 

 base of the arms is shown by the approximation of several of the adambulacralia. IV. In Xenaster 

 margariiatus the arms have fused as far as the third infero-marginal. The incorporated infero-marginalia 

 are still clearly distinguishable. (This figure is reproduced from Text-fig. 32, p. 62, of this monograph.) 



as I hope to show, it serves adequately to separate and illuminate the great mass 

 of the Asterozoa. Let us consider first the growth of the disc in the Asteroidea. 



The most primitive Asteroidea have a very small disc with few plates 



(Section A, Text-figs. 34, 38, and Pis. II, III). In the Hudsonasteridse (Text-fig. 



122a) this disc is so small that the oral surface has only one interradial plate, the 



odontophor, within the axils of the arms. The series of drawings given in the 



text-figure shows that the enlargement of the disc took place by the approxima- 



