274 E. W. MACBRIDE. 



M'liich we may look to find tlie origin of the group Ecbino- 

 dermata as a whole ? 



Ill answer to the first question, we must observe that the 

 stalks of Asterina and Antedon are morphologically equivalent,^ 

 both being formed from the prseoral 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 larvae 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 hydrocoele 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 Las been 

 called to some observations of Perrier's which I regret Laving overlooked. In 

 his account of the Echinoderms collected by the "Mission Scientifique du Cap 

 Horn," he describes the larvae of Asterias spirabilis, which adhere to the 

 buccal membrane of the mother. Tiiey are attached by a pedicle which 

 Perrier compares to the stalk of the Antedon larva and to the praeoral lobe of 

 the Asterina larva. He points out that both in the case of Asterias spira- 

 bilis and ofAsterinagibbosa the pedicle arises from the oral surface, whereas 

 in Antedon it is aboral in its origin, but he ofl'ers no explanation of this dif- 

 ference in position. 



