28 EMBRYOLOGICAL MOXOCUAPHS. 



PLATE IX. 



Development of the Echinoidea. Figures from Emil Selenka, Wilhelm Busch, Johannes 

 MiJLLER, J. W. Fewkes, H. Garman and B. P. Colton, and Alexander Agassiz. 



1-16. Echinus miUaris. From E. Selenka, Keimbliittcr iiml Organ.inlage der Echiniden, 1880, PI. V. Zeits. f. 



Wiss. Zool., XXXIII. 



1. Free swimmiii" Til.istula, optical section ; o, position of the fiituve anus ; b, funnel-shaped depression ; /, seg- 



mental cavity. 16 hours after artificial fecundation. 



2. Blastula with the two clusters of mesodernr cells, tn, m', which have separated from the thickened part of the 



ectoderm ; a, position of the future anus. 18 hours. 



3. Commencement of the invagination. 22 hours. 



4. Young gastrula, optical section; a, gastrula mouth (later anus). 27 hours. 



5. Gastrula, optical section ; small limestone rods have made their appearance : m, digestive cavity ; m, m', accu- 



mulation of cells from which the y-shaped rods take their origin ; c, expansion of the closed e.vtremity of 

 the dio-estive cavity, from which are developed the diverticula forming the water-tubes. 43 hours. 



6. Gastrula, dark mesoderm cells at the upper extremity of the larva; the diverticulum (water-system) of the 



din-cstive cavity forms a T across its closed extremity ; the limestone rods have greatly increased in length 

 and are surrounded by the nomadic skeleton cells. 48 hours. 



7. Gastrula, after iJ4 hours. Optical section, .showing the position of the limestone rods. The digestive cavity, 



B, is becoming differentiated into an esophagus, stomach, and intestine. The water-system, v ji, has com- 

 pletely separated from the digestive cavity. Lettering as before. 



8. 9 10, 11, 12. Successive stages of the digestive cavity and its diverticulum, showing the manner in which the 



water-system is formed as a diverticulum at the blind extremity of the digestive cavity, and how it becomes 

 separated from it. This process takes place, according to Selenka, in less than three quarters of an hour. 



13. Pluteus, after 60 hours. It has lost its cylindrical outline, the rudiments of the arms appear, and the oral and 



anal planes of the larva are developing in opposite directions. 



14. Diagram of the same Plnteus as fig. 13, seen in profile ; a, oesophagus ; ^, stomach ; y, intestine. The skele- 



ton is not indicated. 

 1.1. Pluteus 94 hours after fecundation : o, mouth ; a, anus (gastrula mouth) ; o, oesophagus ; ft stomach ; 7, in- 

 testine ; vp, right water-tube; p, left water-tube, which subsecjuently becomes the water system of the 

 young Echinus and tlie left peritoneal sac. Tlie lesopliagus, a, is capable of a considerable contraction and 

 expansion ; the narrow passages h and i, leading from the resophagus and intestine into the stomach, are 

 well seen in the profile figure of the same Pluteus (tig. 16). 



16. The same Pluteus as fig. 15, seeu in profile. The cilia which still cover the whole Pluteus are not represented 



in this figure, nor in figs. 13, 14. 



17-19. StrongylocaifrolHS Hvitlii.i. From E. Selenka, Keimblatter und Organanlage der Echiniden, 1880, I'l. Vll. 



Zeits. f. Wiss. Zool., XXXIII. 



17. Blastnla m.aking its escape fiom the egg ; d, outer membrane. 



18. The same blastnla, free, optical section ; e a, thickened wall of the blastula at the pole where the invagination 



will take place. 



19. Gastrula 43 hours old. The va.soperitoneal vesicle has separated from the digestive cavity. Optical section, 



the two first j/-shaped limestone rods of the skeleton have appeared. 



20-23. Arhacia puslulosa. From E. Selenka, Keimblatter und Organanlage der Echiniden, 1880, PI. VII. 



Zeits. f. Wiss. Zool., XXXIII. 



20. Blastula 30 liours after artificial fecundation ; c n, thickeued wall of the blastula, where the invagination will 



tiike place ; /, segmental cavity ; </, anal pole. 



