THE METAMORPHOSIS OF ECHINODERMS. 119 
motion may have been, as in the bilateral stage, in its direction. 
However this may be, if fixation, beginning probably with a 
mere sucker, afterwards assumed a more permanent character, 
we have plenty of parallel cases to show the possibility (or even, 
perhaps, probability) that the mouth would move away from 
this point, and eventually reach the opposite pole. Here 
again no certain cause can be assigned; we may suggest that 
this movement was necessary to place the mouth in an advan- 
tageous position for obtaining food ; but if the earlier position 
was disadvantageous, why did fixation ever take place? All we 
can say is that, if we may trust embryology, such a movement 
is a very common sequel to fixation by a point near the mouth. 
The exact position of this sucker in the ancestor of the 
Pelmatozoa I cannot determine with any certainty. It may 
have been anywhere on the oral surface, but we do not even 
know what the extent of this surface was. In the later Cystid 
stages of Antedon (26, pl. x, fig. 90) the mesentery is oblique 
and the oral surface is fairly flat right up to the level of the 
water-pore, and this is true also of young Ophiurids ; while in 
some Cystids, though this surface is not so flat, an equatorial 
line dividing the animal into equal oral and aboral halves 
would lie far on the aboral side of the anus and water-pore, if 
the identification of these apertures can be trusted. But 
without going to such extreme cases as this, it is very easy to 
understand that the hypothetical sucker may have occupied 
almost any position between the transverse mesentery and the 
mouth, though probably not within the atrial cavity. As its 
subsequent migration to the aboral pole does not seem to have 
affected the water-pore, it is probable that it lay nearly over 
the centre of this interradius, and therefore to the right of the 
pore, which, as we have seen, is adradial. But, while thus 
avoiding both water-pore and water-tube, it may possibly have 
involved some portion of the dorsal organ, which, as the diagrams 
show, lay further to the right than either of these structures ; 
or if it was very broad, and was situated at the level of the 
mesentery, it might (though this is less probable) involve in 
its movements the dorsal sac. 
