MEM. M. C. Z., IX. No. 2. — ECHINODERMATA 



43 



PLATE XV. 



Development of Holothuroidea, continued, and Comparison of Echinoderm Larvae. Figures 

 from Emil Selenka and Johannes Muller. 



1 - 13. Cucumaria doliolmn. From E. Si-U'iika, Zur Entwickelung Jer Holothurien (Holotluiria tubulosa uud 

 Cucumaria iloliohim), Ein Beitrag zur Keiinlilattei-tlieorie, 1876, Pis. XL, XII. Zeits f. Wiss. ZooL, XXVII. 



9 



F^). Vasopeiitoneal sac. 



1. Egg found floating on the smface, in process of segmentation, n, genninative vesicle. 



2. Completely segmented blastula. s, the part of the blastoderm where the invagination will take place. 



3. The biastula has become larger, a, position of the future anus ; m, mesoderm cells. 



4. Gastrula at the end of the second day ; longitiulinal section. The migratory cells have accumulated in part 



towards the ectoderm at d, to form later the circular muscular system, and in part remain five in the seg- 

 mental cavity, s, the part of the blastoderm where the invagination to form the digestive cavity can be 

 traced by the slight depression at that pole. 



5. Longitudinal section of an older gastrula. 



G. Sagittal section of larva at the end of the fourth day. 



7. A', £', C, show the mode of formation of the water system, and of the two peritoneal sacs as diverticula 



from the original digestive cavity ; 7t, the poiut where the fully formed invaginated cesophagus strikes the 

 digestive cavity. 



8. Free swimming embryo. |, oil globule of the head. 

 Section of a larva, about in the stage of fig. 8. Lettering as before, with the following additions : — 



U. Circular vascular ring, with the five oral tentacular vesicles. 



F. The five ambulacral canals. 



G. The ventral ambulacral canal, with two ambulacral tentacles. 

 X, Stone canal. 



J. Spheres of food in the stomach. 



K. Anterior lobe of embryo. 



P'. Wall of the peritoneal sac. 



S. Cells with lashes, originating from the Wbratile cord. 



+. Polian vesicle. (This is cut off. ) 



10. Older embryo, swimming freely. 



T. The five oral tentacles, which can be nearly entirely contracted. 



0. The two ventral ambulacral tentacles, with inidimentary sucking disks. 



J. Oil globule in anterior part of the head. 



11. Young Cucumaria creeping. The bands of vibratile cilia have disappeared ; the anterior portion of the young 



Cucumaria is rounded, and the oil globule of the interior has been resorbed. D, ventral ambulacral ten- 

 tacles, with calcareous sucking disks ; S, calcareous plates of the ectoderm. 



