564 INVERTEBKATA chap. 



It follows that the plates termed " radials" and "hasals" in young 

 .Eleutherozoa and Pelmatozoa do not correspond to eaxh other, hut 

 in each case must ie looked on as a rearrangement of scattered plates 

 when the whole lody of the animal had been dominated by pentamerous 

 symmetry. 



Now the change of position which the mouth undergoes in the 

 metamorphosing Orinoid is quite parallel with the change which it 

 undergoes in the larva of a Tunicate, or of an Entoproct after 

 fixation ; and the reason in all three cases is the same, viz. the attempt 

 to bring the mouth into a more favourable position for catching free- 

 swimming prey which fill the water above it. 



What then can have been the motive for the different" shift of the 

 mouth in an Asteroid ? We can only surmise that it was an adapta- 

 tion designed to bring the tentacles which surround the mouth into a 

 more favourable position for grubbing in the mud and detritus on the 

 substratum surrounding the animal. This supposition accords with 

 the fundamental distinction which obtains at the present day between 

 the habits of Eleutherozoa (Asteroidea, Ophiuroidea, Echinoidea, 

 Holothuroidea), which in the majority of cases are scavengers, 

 devouring dead animals and organic detritus lying on the bottom, 

 and those of Pelmatozoa, which to this day feed on Plankton 

 captured by currents produced by the cilia covering their tentacles. 



The difficulty of imagining how a fixed animal could pick up a 

 living by grubbing in the limited area of mud immediately surround- 

 ing it, is easily got over if we, assume that this mud was not 

 motionless, but drifting, if, in other words, the ancestral Asteroid had 

 fixed itself in tide-ways, such as the gaps in coral reefs. Perhaps, 

 indeed, the original purpose of fixation was to enable the animal to 

 resist the pull of these tidal currents and avoid being swept helplessly 

 along. When the old Asteroids extended their range into calmer 

 water, then the breaking of the stalk would set them free to wander 

 about and pick up a living under the new conditions. 



The post-larval history of both Ophiuroids and Echinoids indicates 

 that they are derived from the Asteroid stem. In both cases, after 

 metamorphosis they creep about on their tube feet like an Asteroid, 

 and in the case of the young Echinoid the terminations of the radial 

 canals are free movable tentacles, and the dorsal surface, which later be- 

 comes the insignificant periproct, is larger than the ambulatory surface. 



We can even form a guess as to what led to the evolution of 

 these two orders out of a primitive Asteroid. The Ophiuroidea are 

 merely Asteroidea in which the neuro-muscular system has attained 

 a higher development. In this respect they are the " highest " 

 Echinodermata ; their movements are the most active, and their 

 sensitiveness the most acute. The Echinoidea, on the contrary, seem 

 to be a race of " climbing " Asteroidea. The typical regular urchin 

 loves the vertical faces of stones and the crevices between stones. 

 These give opportunities to employ the tube feet which spring from 

 the upper surface of the body. 



