STUDIES IN THE EMBRYOLOGY OP EOHINODERMS. 445 
even slightly behind it (fig. 17). It is to be noticed that in 
Asterids, as well as in Echinids, the madreporic plate loses its 
relation to the body-cavity, which the other four basals possess, 
as if it were dragged out of its natural position in order to 
embrace the pore. 
Owing to our imperfect knowledge of the development of the 
skeleton in Holothurians we are at present unable to trace in 
this group the relation of the mouth and anus to the radii. 
Enough has, however, been said to make it probable that in 
all groups (except perhaps Holothurians) the radii of the 
abactinal part of the body (including the regions of the right 
and left posterior enterocoels) bear a very definite relation to 
the mouth, anus, and water-pore of the larva ; that, in fact, 
these organs mark out an interradius which, since it contains 
both mouth and anus, might be called ventral, or, as it is an- 
terior to the system of radial plates and contains the prseoral 
lobe (where this is present), may be called anterior. The latter 
term seems to me preferable, since we can with less confusion 
apply it to the adults, though, of course, in seeking for an ante- 
rior interradius in them, we must be guided by the situation 
of the water-pore rather than by the indefinite and variable 
positions of the mouth and anus. 
VOL. XXIX, PART 4. NEW SER. 
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