REPOET ON THE HOLOTHHRIOIDEA. 
5 
ending t}'pically in a suctorial disk; (2) that the nervous system, which is scarcely less 
characteristic of the Eehinoderms, is devoid of ganglia, and consists of a ring round the 
mouth with stems issuin^ from the ring and entirely following the water-vascular 
system. On account of these important coincidences, it appears to me that the common 
progenitor of the three classes ought to have had the above-mentioned organs already 
developed. 
For it should not be assumed, I think, except from particularly cogent reasons, that 
these organs could have been developed so as to agree in every detail, after the three 
classes have diverged from one another. Accordingly, although in the apodous Holo- 
thurids the five radial ambulacral vessels are either totally wanting (in the Synaptidse 
and some Molpadidm), or, if existing, are without any connection wdiatever with the feet 
(in the rest of the Molpadidse), it seems to me much more probable that this organisation 
of the Apoda is to be regarded as the result of a reduction than as an original condition. 
If a >Sy?2«^9^«-shaped animal should be regarded as the common progenitor, the present 
apodous Holothurids must of course be considered to be its least altered descendants. 
Then we have to choose betvN'cen two possibilities. There may have issued from this 
main stem a form that I13' developing amlmlacral canals with feet and ampullse has 
become the progenitor of the pedate Holothurids, the Echinids and the Asterids. In 
this case we arrive at the conclusion that the pedate Holothurids are more nearly related 
to the Echinids and Asterids than to the Apoda. The other possibility is that the 
pedate Holotliurids on the one hand, and the Echinids and Asterids on the other, have 
l)ranched oft’ from the common main axis of the Eehinoderms, and, independently of each 
other, developed feet and radial ambulacral vessels similar in detail. Although in this 
latter case the relationship between the Pedata and Apoda becomes more evident, still 
there remains the almve-mentioned difficulty of understanding how such a comj^licated 
water-vascular system so similar in detail could have arisen, notwithstanding that the 
classes had developed independently of each other. On this account I think it more 
probable that the primitive form of tlie Holothurioidea has been a pedate animal, that 
the Synaptidse represent their most diff’erentiated form, and that in the Molpadidse the 
reduction of the water-vascular system is still going on. 
If, on the other hand, the derivation of the Holothurioidea is to be founded on 
Ontogeny, it would follow that the Elasipoda might justly claim to be regarded as being 
phylogenetically the oldest, l:)ecause they have for the most part maintained the connec- 
tion of the stone-canal with the exterior, and present such numerous instances of an 
extremely peculiar calcareous ring composed of spicules, and strongly reminding one of 
that of the larvae, and because they have, moreover, in general, very primitive deposits 
in tlie perisoma, and have thus preserved many important peculiarities no doubt 
l)elongingto the common ancestors, but lost by the other Holothurioidea. The Elasipoda 
have, indeed, become much changed in other respects from the primitive type, as is 
