MEM. M. C. Z., IX. No. 2. —ECHINODERMATA. 4] 
PLATE XIV. 
Development of HotorHuRoIDEA, continued. Figures from Emit Sevenka, and D. C. Dantets- 
SEN and J. Koren. 
1-12. Holothuria tubulosa. From E. Selenka, Zur Entwickelung der Holothurien (Holothuria tubulosa und 
bo 
10. 
Vale 
12. 
. The transparent larva, seventy-one hours after fecundation. sf, vertical axis of the larva when swimming 
Cucumaria doliolum), Ein Beitrag zur Keimblattertheorie, 1876, Pls. IX., X.  Zeits. f. Wiss. Zool., XX VII. 
a, Anal opening. A. (Esophagus. 
61. Blastoderm. B. That part of the digestive cavity from which 
ek. Ectoderm. the vasoperitoneal sac has separated. 
en. Entoderm. H. Stone canal. 
f. Segmental cavity. N. Nucleus. 
m. Mesoderm. P. Peritoneal sac. 
m!. Exterior mesodermal plate. Pr. Right peritoneal sac. 
m'!, Interior mesodermal plate. Pl. Left peritoneal sac. 
o. Mouth. u. Original digestive cavity of larva. 
w. Cord of vibratile cilia. V. Water-system sac. 
x. Water-system pore. Vp. Vasoperitoneal sae. 
. Six hours after fecundation. Optical section. a, nucleolus ; a, membrane of egg. 
. Blastula, fifteen hours after segmentation ; the segmentation is nearly complete. s, spermatozodn ; mu, mi- 
cropyle. Many of the blastoderm cells already have a ciliated lash. The blastoderm rotates slowly and 
irregularly within the egg membrane. 
. Gastrula, twenty-three hours after fecundation ; longitudinal optical section. Commencement of the invagina- 
tion ; a few cells, the mesoderm, have separated from the ectoderm ; the egg membrane has been ruptured, 
_and has disappeared. 
. A transparent larva, seen from the ventral side, forty-four hours after fecundation ; the digestive cavity, w, 
already shows a constriction, the point of subsequent separation of the vasoperitoneal sac from the distal 
extremity of the digestive cavity. R, green bodies of the vibratile cord containing cells. 
. Optical section of a larva, fifty-one hours after segmentation ; the vasoperitoneal sac, Vp, has become sepa- 
rated from the anal part of the digestive cavity, B. 
. Diagrammatic profile view of the same larva. Formation of an atrium, 4, the future esophagus, by inva- 
gination of the blastoderm. The vibratile cords and mesoderm cells have not been drawn. 
. Optical section of a larva, sixty-nine hours after segmentation, seen from the dorsal side. 4, blood cells in the 
vasoperitoneal sac; #?, the green granules, containing cells of the vibratile cord; p, contents of the 
digestive cavity. The cesophagus and intestine are now connected ; the vasoperitoneal sac is pushed to 
the left side. 
oS 
in a natural attitude, and round which it slowly rotates, moving forward in long spirals at the same time. 
. The transparent larva, a hundred hours after the feeundation. The vasoperitoneal sac has divided into three 
distinct sacs, the water-system sac, and the two lateral disks. 
Diagrammatic profile view of fig. 8. 
Diagrammatic profile view of fig. 9 ; a few mesoderm cells, g, round the stone canal, x, have been indicated. 
Sketch of the digestive cavity and the surrounding parts of the same larva (fig. 9), a few hours later (seven 
hours). 
13-27. Holothuria tremula. From D. C. Danielssen and J. Koren, Observations sur le Développement des 
Holothuries, 1856, Pls. VII., VIII. In Fauna littoralis Norvegie, par Dr. M. Sars, J. Koren, et D. C. 
Danielssen, Seconde Livraison, 1856, Pls. VII., VIII. 
13. Young embryo, recently hatched ; still covered with vibratile cilia. 
14. Somewhat older embryo, with a mouth opening, a. 
