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



17 



PLATE V. 



Development of Asteroidea. Figures from Wilhelm Busch and Hubert Ludwig. 



1-4. Echinaslcr sepositus. From W. Biisch, Beobachtungen iiber Anatoniie und Entwickeliing einiger Wirbel- 



losen Seetbiere. Berlin, 1851, PI. XII. 



1. Young pelagic embryo : o, body ; b b, so-called bracliiolarian appendages. 



2. The .same embryo somewhat more advanced : a, the body where the future Starfish is developed ; b I, so-called 



bracliiolarian appendages ; o, commencement of a third pair. 



3. The young starfish has assumed a pentagonal outline ; the tentacles (6) of the disk are clearly indicated, and 



the brachiolarian appendages have taken their maximum development on each side of the a.vis a. 



4. The most advanced stage of the young Echinaster observed by ICrohn. The brachiolarian appendages are 



reduced by resorption to mere rudiments, a ; b, older pair of tentacles ; d, odd terminal tentacles ; v, young- 

 est pairs of tentacles ; /, actinal amliulacral furrow. 



5 ~ 49. Asterina gihbosa. From 11. 



Ludwig, Morphologische Studieu an Echinodermen, II., 2 Heft, 1882. Zeits. 

 f. Wiss. Zool., XXXVII., Pis. L-VI. 



L s. Larval ccsophagus. 



M. Mesentery. 



Ms. Mesoderm. 



P. Dorsal pore. 



r El . Right euterocadum pouch. 



?!, T^, T3, T^, T^. First to fifth terminal plates. 



1, 2, 3, 4, 5, denote the Hydroccelum lobes and am- 

 bulacral arm lobes. 



/, II, 111, IV, V, denote the antiambulacral arm 

 lobes; denotes the upper side of the embryo 

 or larva ; U, the lower ; H, the posterior, and 

 V, the anterior extremity ; L, left ; R, right; 

 V L, in front to the left; H U, behind and 

 below. 



First to fifth Interradial. 



5-10. Stages of segmentation of the first day. 



5. Three spheres of segmentation are shown. I, I, are formed from the division of the u]iper of the two spheres ; 



II, the lower sphere ; this is originally somewhat larger than the upper sphere. 



6. The lower .sphere, II, is now also divided into two, IIj . . . . 



7. The same as fig. 6, so turned as to show the two spheres Ij . . . . 



8. The stage with four spheres somewhat older than the preceding figures. 



9. A stage with eight spheres ; I2 and II2, the two splieres resulting from the division of the spheres I ami II 



of the stage of fig. 8. 



10. Stage with sixteen spheres, which have resulted from the subdivision of the cells \., into I3, and of the cells 



II2 into II3. 



11. Section through a blastula composed of 32 spheres. 



12. Longitudinal section of a gastrula on the second day. Om, the gastrula mouth. 



13. The same stage, looking into the gastrula mouth. 



14. Longitudinal section of an older gastrula, with a decidedly narrower opening, Gm. 



15. 16, 17. Posterior, anterior, and right view of an embryo just escaped from the egg, fourth day. 



18. Longitudinal section of a gastrula somewhat older than fig. 14 ; the gastrula mouth is approaching the lower 

 pole : a, the point at which the diverticula of the gastrula digestive cavity begin to be formed. 



