NOTES ON ECHINODEKM MOKPHOLOCJ Y. 375 



the two peritoneal caeca is just in front of the closed blastopore, 

 between it and the position of the permanent mouth ; and it is 

 not improbable that it contains a blood-vascular space like that 

 described by Ludwig in the Asterid larva. The blastopore marks 

 the position of the anus of the Crinoid, and it is just between the 

 mouth and anus of Actmometra that the labial plexus is chiefly 

 developed, in which the genital vessels originate.^ This labial 

 plexus is really circumoral, concentric with the oral blood- 

 vascular ring, but farther from the gullet ; and the above con- 

 siderations tend to support the suggestion which I ventured to 

 put forward last year respecting the homology of this plexus 

 with the aboral ring described by Ludwig in Starfishes and 

 Ophiurids. If this be the case the central plexus of the Asterid 

 is much shortened in the Crinoid, but expanded laterally instead. 

 For the labial plexus and oral ring are in communication all 

 round the gullet. Future observations on the development of 

 the blood-vascular system of the Crinoids may, perhaps, show 

 that there is primitively only one connection between the two, 

 and that on the right of the primary water-tube, i.e. in the 

 direction of the anus, between which and the mouth the labial 

 plexus and its connection with the oral ring are chiefly developed 

 in the adult. 



Hence in the Crinoid the aboral blood-vascular ring would be 

 on the ventral side, while it is dorsal in the Asterids. It is very 

 curious that, according to Ludwig's researches on the Ophiurids, 

 the aboral ring {pace Apostolides) actually is on the ventral side of 

 the disc in the interradial areas; while on the right of the water-tube 

 there is a shortened central plexus connecting it with the oral 

 ring ; and, lastly, that the aboral ring contains a cellular genital 

 cord, like that within the genital vessels which originate in the 

 labial plexus of the Crinoids. 



The observations of Jourdain'^ and Greeff^ possibly indicate 

 the existence of the same structure within the aboral ring of the 

 Asterids. In the relations of the blood- vascular system, there- 

 fore, as in so many other respects, the Ophiurids seem to stand 

 midway between the Asterids and the Crinoids. 



Four years ago"* I ventured to suggest some modifications in 

 the current views respecting the homologies between the calyx 

 plates of a Crinoid and those in the apical system of an Urchin ; 

 and at the same time I endeavoured to replace the irregular and 

 empirical nomenclature then in use for the plates of the Crinoidal 

 calyx by one of a more rational kind. By this I mean one which 



1 This Journal, vol. xxi, 1881. pp. 184, 185, PI. xii, fig. 14. 



2 ' Marburg Sitzungsbericlite,' 1872, No. 11, p. 164. 

 ^ ' Comptes Rendus,' t. xciv, pp. 744-746. 



'' This Jourual, vol. xviii, 1878, pp. 352-383. 



