MEM. M. C. Z., IX. No. 2. — ECHINODEEMATA. 21 



36. Embryo in which the l.arral organs, a a, are foriiiing. 



37. Soniewliat older embryo. 



38. The young Starfish is well advanced : c, ambulacral tentacles ; the hrachiolarian appendages, a a, are fully 



developed, and a small tubercle, b, has formed on the upper surface of the larval organ. Seen from the 

 actinal side. 



39. The same as fig. 38, seen I'rom the abactinal side. 



40. Somewhat older stage, seen from the actinal side ; the young Starfish has now assumed a well-defined pentago- 



nal outline. 



41. The same embryo as fig. 40, seen from the abactinal side. 



42. The same embryo, about two months old, seen from the abactinal side after the resorption of the larval organs. 



43-46. Echinastcr Sarsii. From J. Miiller, Ueber den Allgemeinen Plan in der EntwiclceUing der Echinoder- 

 men (Pt. 6), 1852, PI. I. Abhandl. der K. Akad. der Wiss. Berlin, IS53. 



a. Larval organ, odd hrachiolarian arm. /. Interior cavity. 



b. Larval organ, paired hrachiolarian arms. /'. Cavity leading to the brachiular appendages. 



c. Tubercle between the hrachiolarian arms. g. Digestive cavity. 



d. Cortical layei'. !i. Interior of the digestive cavity. 

 c. Inner layer. i. Tentacles of the Starfish. 



43. Longitudinal section through an embryo. 



44. Transversely longitudinal section across the brachiolaiian arms. 



45. Longitudinal sections of an older embryo in which the body cavity is divided into two, one of wliich contains 



the digestive cavity, and the other communicates with the larval organs. 



46. A similar section through an embryo somewhat older than fig. 45. 



47. 48. Wurmforinige Asteric. From .1. Miiller, Ueber den Allgemeinen Plan in der Entwickelung der Echino- 



dcrmcn (Pt. 6), 1852, PI. I. Quoted above. 



a. Lateral ambulacral tentacles. b. Odd terminal tentacle. c. Spines of the actinal surface. 



47. IVurmformige Astcric, seen from the abactinal side. 



48. The same, seen from the actinal side. 



49-51. Pteraster militaris. From J. Koren .and D. C. Danielssen, Observations sur le Developpement des Aste- 

 ries, in Fauna littoralis Norvegiffi, Seconde Livraison, 1856, PI. VIII. 



a. Anal opening. d. Circular water canal. 



b. Intestinal canal. e. Madreporic canal. 



c. Extremity of the intestinal canal. 



49. Young embryo. 



50. Young embryo which has assumed a ]ientagonal outline. 



51. A young Starfish seen from the actinal side ; b, ambulacral plates ; c, tentacles. 



Figs. 49, 50 are much younger than the stages figured by Sars ; see this Plate, figs. 20-23. Fig. 51 is about 

 in the stage of fig. 22. 



