MEM. M. C. Z., IX. Ko. 2.— ECHINODERMATA. 41 



PLATE XIV. 



Development of Holothuroidea, continued. Figures from Emil Selenka, and D. C. Daniels- 



.SEN and J. KoREN. 



1-12. Holoihuria tuhulosn. From E. Selenko, Zur Entwiclcelung der Holotluirien (Holotlmria tubulosa uud 

 Cucumaria doliolum), Ein B.'itrag zur Kcimblattertheorie, 1876, Pis. I.X., X. Zeits. f. Wiss. ZooL, XXVII. 



A. (Esojihagus. 



B. That part of the digestive cavity from which 

 the vasoperitoneal sac has separated. 



H. Stone canal. 



N. Nucleus. 



P. Peritoneal sac. 



Pr. Right peritoneal sac. 



PI . Eeft peritoneal sac. 



u. Original digestive cavity of larva. 



v. Water-system sac. 



V2'- Vasoperitoneal sac. 



1. Six hours after fecundation. Optical section. «, nucleolus ; a, membrane of egg. 



2. Blastula, fifteen hours after segmentation ; tlie segmentation is nearly complete, s, spermatozoon ; m, mi- 



cropyle. Many of the blastoderm cells already have a ciliated lash. The blastoderm rotates slowly and 

 irregularly within the egg membrane. 



3. Gastrula, twenty-three hours after fecundation ; longitudinal optical section. Commencement of the invagina- 



tion ; a few cells, the mesoderm, have separated from the ectodenu ; the egg membrane has been ruptured, 

 and has disappeared. 



4. A transparent larva, seen from the ventral side, forty-four hours after fecundation ; the digestive cavity, u, 



already shows a constriction, the point of subseipient separation of the vasoperitoneal sac from the distal 

 e.xtremity of the digestive cavity. E, green bodies of the vibratile cord containing cells. 



5. Optical section of a larva, fifty-one liours after segmentation ; the vasoperitoneal sac, Vp, has become sepa- 



rated from the anal part of the digestive cavity, B. 



6. Diagrammatic profile view of the same larva. Formation of an atrium, A, the future oesophagus, by inva- 



gination of the blastodei-m. Tlie vibratile cords and mesoderm cells have not been drawn. 



7. Optical section of a larva, sixty-nine honrs after segmentation, seen from the dorsal side, b, blood cells in the 



vasoperitoneal sac ; R, the green granules, containing cells of the vibratile cord ; p, contents of the 

 digestive cavity. The oesophagus and intestine are now connected ; the vasoperitoneal sac is pushed to 

 the left side. 



8. The transparent larva, seventy-one hours after fecundation, sf, vertical axis of the larva when swinnning 



in a natural attitude, and round which it slowly rotates, moving forward in long spirals at tlie same time. 



9. The transparent larva, a hundred hours after the fecundation. The vasoperitoneal sac has divided into three 



distinct sacs, the water-system sac, and the two lateral disks. 



10. Diagi-anmiatic profile view of fig. 8. 



11. Diagrammatic profile view of fig. 9 ; a few mesoderm cells, g, round the stone canal, a-, have been indicated. 



12. Sketch of the digestive cavity and the surrounding parts of the same larva (fig. 9), a few hours later (seven 



hours). 



13-27. Uolothiiria tremuJa. From D. C. Danielssen and ,T. Koren, ..Observations sur le Developpement des 

 Holothuries, 1856, Pis. VII., VIII. In Fauna littoralis Xorvegiffi, par Dr. M. Sars, J. Koren, et D. 0. 

 Danielssen, Seconde Livraison, 1856, Pis. VII., VIII. 



13. Young embryo, recently hatched ; still covered with vibratile cilia. 



14. Somewhat older embryo, with a mouth opening, a. 



