DEVELOPMENT OF CUCUMARIA. 639 



to the radii, but the disposition is totally difterent, and in addition 

 the Polian vesicle is in the left ventral interi'adius. The two agree 

 in one respect only : the tentacles alternate ivitli the radial canals. 

 Edwards did not attend, apparently, to the place of actual origin 

 of the tentacles, and a clearing up of that point would be valuable. 



The present paper, demonstrating, I believe, that there is a 

 primary alternation of tentacles and radial canals in two species 

 of Cucumaria, seems to support the assumption made by Semon, 

 whose phylogenetic speculations, however, I am not concei'ned to 

 defend. 



The only other feature of the fourth-day larva to which I 

 shall allude is the condition of the stone-canal. In the position 

 where, on the third day, a slight dilatation was observed, there 

 is now an area of the antero-dorsal wall of the canal in which 

 the cells have become large and clear, the cells of the other walls 

 of the canal remaining columnar and closely packed. The 

 appearance of cross-sections through this region is now very 

 characteristic (PI. II. fig. 14) owing to the crowded, darkly- 

 staining nuclei in the postero-ventral and lateral walls of the 

 tube. Examination of later larvse proves that Ave have in this 

 swelling-up of certain cells of the stone-canal a preliminary stage 

 in the thinning-out of the part affected to form the vesicle 

 noticed by Bury (2), Ludwig (7), and MacBride (8 & 9), which is 

 convertecl into the internal maclreporite of the adult. It will be 

 clear from inv description of its mode of origin that Bury's view 

 that this vesicle represented the anterior coelom is incoriect. It 

 is, on the contrary, a secondary difterentiation of the stone-canal. 

 In this conclusion I agree with Ludwig. 



References to Literature. 



(1) Bury, H. (1895).— "The Metamorphosis of Echinoderms," 



Quart. Journ. Micr. Sc. London, vol. xxxviii. pp. 45-135, 

 pis. 3-9. 



(2) Bury, H. (1889).— " Studies in the Embryology of the 



Echinoderms," Quart. Journ. Micr. Sc. London, vol. xxix. 

 pp. 409-449, pis. 37-39. 



(3) Clark, H. L. (1898). — '•'• Synapta vivipara: A Contribution 



to the Morphology of Echinoderms," Boston, Mass., Mem, 

 Soc. Nat. Hist. vol. v. pp. 53-88, pis. 11-15. 



(4) Edwards, C. L. (1909).' — "The Development of Holothuria 



floridava Pourtales, with special reference to the Ambu- 

 lacra! Appendages," Journ. Morph. vol. xx. pp. 211-230, 

 pis. 1-3. 



(5) Gemmill, J. F. (1912).— "The Development of the Starfish, 



Bolaster end,eca Forbes," London, Ti-ans. Zool. Soc. vol. xx. 

 pt. 1, pp. 1-71, pis. 1-5. 



(6) Gemmill, J. F. (1915). — " The Larva of the Starfish Porania 



pulvillus (0.. F. M.)," Quart. Journ. Micr. Sc. London, 

 vol. ixi. pp. 27-50, pis. 4 & 5. 



