392 E. W. MACBRIDE. 
which we may look to find the origin of the group Echino- 
dermata as a whole? 
In answer to the first question, we must observe that the 
stalks of Asterina and Antedon are morphologically equivalent,! 
both being formed from the preoral lobe, and, so far as one 
might judge from the different shape of the latter in the two 
cases, the adhesive discs by which they fix themselves are 
situated in precisely the same position. Now no one doubts 
that Antedon had a fixed ancestor; it is, in fact, one of the 
very few Crinoids which do not remain fixed throughout their 
whole life. If Asterids ever had an ancestor in common with 
Crinoids which could be called an Echinoderm at all, it must 
have been one represented by the fixed larva of Antedon before 
it has fully acquired radial symmetry, since, as we have already 
pointed out, the metamorphoses of Antedon and Asterina 
pursue different courses. In the first case the mouth is shifted 
backwards and upwards, and a precisely similar thing happens 
to the larve of Entoproct Polyzoa, Ascidians, and Cirri- 
pedes when they fix themselves. In the second case, how- 
ever, the disc is flexed obliquely downwards on the stalk, so that 
the left coelomic sac and the hydroceele both come to encircle 
the base of the stalk ; and as consequence the aboral poles in 
the two cases are not homologous, for in the first case this pole 
is the cicatrice left by the rupture of the stalk, whereas in the 
second case the point where the stalk passes into the disc is 
quite remote from the aboral pole. The apparent correspondence 
of the calcareous plates of the calyx in Antedon and the so- 
called calyx in Asterina is simply due, in my opinion, to the 
' Since the present paper was sent in for publication, my attention has been 
called to some observations of Perrier’s which I regret having overlooked. In 
his account of the Echinoderms collected by the “‘ Mission Scientifique du Cap 
Horn,” he describes the larve of Asterias spirabilis, which adhere to the 
buccal membrane of the mother. They are attached by a pedicle which 
Perrier compares to the stalk of the Antedon larva and to the preoral lobe of 
the Asterina larva. He points out that both in the case of Asterias spira- 
bilis and of Asterina gibbosa the pedicle arises fromthe oral surface, whereas 
in Antedon it is aboral in its origin, but he offers no explanation of this dif- 
ference in position. 
