THE METAMORPHOSIS OF ECHINODERMS. 109 
only been found in the larve of two groups—Asterids (both 
forms of larvee) and Echinids, but I shall assume for the present 
that it has a phylogenetic value. Situated, as it is in these 
larvee and in my hypothetical ancestor, just to the right of the 
water-pore, it seems, like the latter, to have escaped the move- 
ment into the left side, or perhaps I ought to say, has moved 
with the pore and the anterior end of the stomach into an 
equatorial position. A glance at my diagrams will show that 
what was formerly on the right side of the water-pore must 
now be on its aboral side; and this is precisely the position 
of this sac in adult Asterids and Kchinids. 
The axial organ was assumed to run forward along the dorsal 
surface of the oesophagus, and it seems to have accompanied 
the latter in its change of position relative to the stomach and 
mesentery. As it lay to the right of the water-tube in the 
bilateral stage, so now it still occupies the same relative position 
when the animal is viewed from the aboral side with the inter- 
radius of the water-pore directed away from the observer. 
One end of the dorsal organ, however, was assumed to lie under 
the dorsal sac; if it retained this position it must, in the 
pentamerous stage, pass round at the aboral end of the water- 
tube from the right side of the latter to the aboral side of the 
water-pore, and this is precisely the course which, as 
Ludwig has shown (18, p. 159) it follows in adult 
Asterids. It seems to me that, apart from any possible 
homologies with Enteropneusta, it is not an unimportant feature 
of my hypothesis that it enables us to derive this very peculiar 
and asymmetrical course from an originally symmetrical one ; 
and I believe that, if ever the axial organ is discovered in the 
bilateral stage of an Ophiurid Pluteus, it will be found to 
occupy exactly the position ascribed to it in my bilateral 
ancestor. 
In fig. 45 I gave a forward extension to the anterior body- 
cavity, in order that it, with the water-tube and dorsal organ, 
might be brought down into the left side, and there by en- 
veloping these two structures form the “axial sinus.” That 
it might be made to do so is sufficiently obvious; but until 
