THE METAMORPHOSIS OF ECHINODERMS. 129 
boscis gland.” It consists, according to Spengel, of a number 
of folds of the epithelium of the proboscis cavity (anterior 
body-cavity), and contains a number of cells with yellow 
granules—apparently excretory; it is also supplied with 
blood by a number of lacune. 
Now I believe that we can recognise all these structures in 
Echinoderms, though the metamorphic changes which the 
larvee undergo greatly obscures their true position. In Asterid 
and Echinid larve we find on the dorsal side, just over the 
junction of stomach and esophagus, and to the right of the 
water-pore, a closed vesicle of schizoceel origin. The floor of 
this ‘‘ dorsal sac,”’ as I have called it, is raised up in Echinid 
Plutei, and in these Jarve (and occasionally in Asterids) a 
pulsation may be observed—not of the vesicle as a whole, but 
apparently of its floor only. 
Besides this, we have in all Echinoderms (except Holo- 
thurians) a glandular organ known as the “ dorsal organ,” 
‘‘axial organ,” “ovoid gland,” &c. The origin of this is 
somewhat obscure. In Crinoids and apparently in Ophiurids 
(18) it is at first a solid mass of cells; but in Asterids (Asterina) 
MacBride describes it as “an ingrowth of the left posterior 
celom into the septum separating the posterior coelomic cavities 
from the axial sinus” (19, p. 433), that is to say, from the 
anterior enterocel. I have not been able fully to satisfy myself 
on this point either in Bipinnaria or Brachiolaria, but I would 
point out that, owing to the fusion of the two cavities (anterior 
and posterior), it must be almost impossible to determine with 
certainty from which of them the organ in question is derived. 
But whatever may be the case in the larva, its structure and 
relations in the adult are very striking. In all forms it isa 
much folded mass of apparently excretory cells, and in Asterids 
and Kchinids, at any rate, if not in Ophiurids (Cuénot and 
MacBride disagree on this point) it projects into the anterior 
body-cavity (axial canal) in such a way that its folds may 
be said to be involutions of this cavity. 
It seems to me that we have a good prima facie case 
in favour of the homology of the “ dorsal sac” and “ dorsal 
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