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E. W. MACBRIDE. 
of the stomach in fig. 1. For such a larva the term “enan- 
tio m or ph ous ” has been suggested. I have described 
enantiomorphous larvae of Asterina gibbosa and of 
Ophiothrix fragilis, but this is the first time that such a 
larva has been recorded in the case of a species of Echinoid. 
This larva, then, affords a proof — if proof were needed — that 
the view which I put forward as to the homology of the 
madreporic vesicle or dorsal sac is correct. If, then, the larva 
in the three groups of the Asteroidea, Ophiuroidea and 
Echinoidea carries a vestigial right antimere of the hydrocoele, 
it is a fair conclusion that in the free-swimming ancestor of the 
Echinodermata the hydrocoele was a paired structure. But 
the double hydrocoele, equally developed on each side, was lost 
as soon as the free-swimming habit was given up and the fixed 
stage had been attained, for it was almost certainly due to the 
atrophy of the right hydrocoele that the left hydrocoele, from 
being a hoop, became a ring. Now we have no reason whatever 
to believe that the primitive bilateral ancestor of Echiuo- 
dermata possessed an amniotic cavity; this is a peculiarity 
of Echinoidea, for in both Asteroidea and Ophiuroidea 
the skin covering the hydrocoele is flush with the general 
surface of the body, and this is obviously a more primitive 
condition than to have an amnion. The interest, therefore, in 
our abnormal larva deepens, for though the double hydrocoele 
is an ancestral atavistic feature, the double amniotic cavity 
cannot possibly be. Now if we had not such larvae as the 
one which we are considering to guide us and if we were 
endeavouring to unravel the factors in normal development, 
we should note the fact that the hydrocoele is formed before 
a trace of the amniotic cavity is visible. We should 
therefore conclude either that the amnion is due to an 
influence on the ectoderm emanating from the hydrocoele, or 
that there is a spot in the ectoderm predestined to form the 
amnion, though we cannot distinguish it from other spots with 
the naked eye. Our larva, however, leads us to decide in 
favour of the first of these two alternatives, and we are thus 
led to the conclusion that the formation of the ectodermic 
