THE METAMORPHOSIS OF ECHINODERMS. 125 
which cannot well be illustrated by figures in two dimensions, 
but will be found by those who take the trouble to con- 
struct models, to offer interesting evidence in favour of the 
view here advanced. In fact, the only obstacle to this view 
with which I am acquainted lies in the unusual position of 
the water-pore on the right, instead of as usual on the left, of 
interradius A (see fig. 14); but this is equally puzzling on any 
other hypothesis, except Semon’s untenable one that the five 
tentacles of Auricularia are radial; and indeed, as already 
suggested, the difficulty may be more apparent than real, 
being perhaps due to the precocious development of the ten- 
tacles of radius I, while one of those belonging to radius V 
only appears later (see 16, p. 183). 
The further question of the exact stage at which the Holo- 
thurians branched off from the parent stock, is not one on 
which I care to express any very decided opinion. I would 
point out, however, that it is difficult to conceive of any torsion 
of the water-vascular ring occurring after the radial canals had 
spread far over the disc and become united with parts of the 
body-wall; and consequently I am inclined to the belief that 
the separation of this class occurred very early, perhaps even 
before that of the Pelmatozoa. The possibly primitive character 
of the genital organs in the Elasipoda fits in very well with 
this supposition ; while the fact that the ambulacral fields are 
limited to the region of the left body-cavity in other Echino- 
derms, but run to the extreme posterior end regardless of the 
body-cavities in Holothurians, is, so far as it goes, opposed to 
the derivation of the latter from any of the other groups of the 
Kchinozoa. 
Relation of Echinodermata to Enteropneusta. 
Of the various features which have from time to time been 
supposed to show affinities between the Echinodermata and 
Enteropneusta, probably the least important, though one of the 
first to attract attention, is the outward resemblance of certain 
Echinoderm larve to Tornaria. No one who has seen the latter 
and Auricularia alive can fail to be struck with their general 
