VII. Morphology and Bionomics. 1915 212 



the Edrioasterid organization, first considering what the known facts 

 of skeletal structure imply as to the general anatomy and mode of 

 life, secondly comparing the structure with that of other echinoderms, 

 especially the Asteroidea. Although it may he necessary to refer to 

 various facts that fall to he dealt with in future Studies, still the 

 present seems a convenient opportunity for this general discussion, 

 because of the recent publication by Mr. W. K. Spencer of the 

 introduction to " A Monograph of the British Palaeozoic Asterozoa" 

 (Feb., 1914, Palaeontogr. Soc. vol. for 1913), by Dr. J. F. Gemmill of 

 his highly important memoirs on the starfishes Solaster endeca (1911, 

 Proc. R. Phys. Soc. Edinburgh, vol. 18, pp. 174-91 ; and Feb., 

 1912, Trans. Zool. Soc. London, vol. 20, pp. 1—71, pis. i-v) and 

 Asterias rubens (Oct., 1914, Phil. Trans., ser. B., vol. 205, pp. 213-94, 

 pis. xviii-xxiv, and March, 1915, Proc. Zool. Soc. London, pp. 1-19, 

 pis. i-iii), and by Dr. A. F. Foerste of some suggestive " Notes on 

 Agelacrinidre " (Sept., 1914, Bull. Sci. Lab. Denison Univ., vol. 17, 

 pp. 399-486, pis. i-vi). 



First, then, as to the internal organization and mode of life of the 

 Edrioasteridae. It is clear from the position of the three openings — 

 mouth, anus, and water-pore — on one face, that that face was directed 

 upwards as in normal Pelmatozoa. Apart from that argument, the 

 general resemblance of Edrioaster and Binocystis to the undoubtedly 

 sessile A gelacrinidae clearly indicates a similar position with regard 

 to the sea-floor, even though the edrioasterids in question may not 

 have been permanently attached. Steganoblastus obviously was fixed 

 by a stem of ordinary pelmatozoan character, and by the mechanical 

 stresses thus set up its theca has been modified so as to present 

 a remarkable resemblance to that of a blastoid. How this stem 

 originated is suggested by the parallel history of Pyrgocystis, in which 

 we seem to trace the gradual elevation of the loosely and irregularly 

 plated thecal wall, as seen in Cystaster, through the low turret of 

 P. sardesoni, to the elongate stem-like turret of P. grayae and the more 

 regularly plated turret of P. ansticei and P. sulcata. 



Another criterion of Pelmatozoa is: "Food brought to the mouth 

 by a subvective system of ciliated grooves, radiating from the mouth." 

 The evidence for this in all Edrioasteroidea is the presence of radiating 

 grooves protected by cover-plates, which plates are particularly well 

 developed in Edrioasteridae and Steganoblastus, and apparently 

 immovable over the mouth-region. 



The only pelmatozoan character not yet mentioned is the presence 

 of " an aborally placed motor nerve-centre". The presence of such 

 a centre in Steganoblastus at any rate may be inferred from the stem 

 with its lumen (Study V, 1914, p. 202) and from the axial folds 

 similar to those which in many crinoids are known to have afforded 

 passage for the nerves from that centre (Study Y, p. 197). The 

 possible relation of the lobes seen on the adapical face of Edrioaster 

 to a chambered organ, such as that with which this nerve-centre is 

 connected in the crinoids, was discussed in Study II (1900, p. 202). 

 It is, however, scarcely necessary to point out that a sessile or almost 

 sessile form such as Edrioaster, without stem and without movable 

 arms, had little or no need for such a motor nerve-centre. If we 



