48 



these groups the body form must have become considerably 

 changed. It must have been flattened antero-posteriorly, so 

 as to reduce the oro-anal axis, and then pushed out radially 

 along the lines of the chief parts of the water-vascular 

 system, so as to produce, first a pentagonal, and then a 

 stellate shape. There being then only half the body in a 

 position for creeping, the water- vascular system was developed 

 only on the lower (oral) surface, in place of extending almost 

 to the aboral pole, as in Holothurians and Echinoids. 



From such an ancestor the Asteroidea were probably 

 given off, and they have retained all the essential characters, 

 while some of their systems have become more highly 

 differentiated. In the Ophiuroidea and the Crinoidea the 

 radial processes of the body or arms have become more 

 marked, and the formation of calcareous matter has increased 

 greatly, the result being that the arms are almost entirely 

 solid, and in great part formed of calcareous ossicles. These 

 changes took place after the separation of the ancestral 

 Asteroids, so that probably the Ophiuroids and Crinoids 

 have had a rather longer common ancestry. 



The ancestral Crinoids, after the Ophiuroids diverged, 

 then probably became fixed by the aboral end of the body, 

 and this necessitated a change in the position of the anus, 

 resulting in both openings of the ahmentary canal coming 

 to lie upon the same surface of the body. The extinct 

 Cystoidea and Blastoidea may be placed as neighbouring 

 side branches from the primitive Crinoids. Probably these 

 three groups are all somewhat degenerate. 



The remarkable characteristics acquired by most Echino- 

 derms during their larval stages (e.g., Pluteus) are certainly 

 merely adaptive and have no phylogenetic significance, and 

 this explains the sudden, and in some cases, very profound 

 changes which may take place when the animal throws off its 

 temporary larval characteristics and acquires its adult structure. 



