MEM. M. C. Z, IX. No. 2. — ECHINODERMATA. 15 



PLATE IV. 



Development of Ophiuroidea, continued. Figures from Nicolas Christo ApostolidIis and 



Johannes Muller. 



1-13. OpJiiothrix versicolor. From N. C. .Apostolides, Anatomie et DeVeloiipement dcs Ophiures. Archives de 



Zool. Exp et Gen., X., PI. XI., 1881. 



1. Egg. m, outer envelope ; y, yolk ; r, geraiinative vesicle ; n, micleus. 



2, 3, 4, 5. Different stages of segmentation. 



6. Blastosphere. 



7. Blastosphere. e, ectoderm cells. 



8. Older than fig. 7. c, ectoderm; y, yolk cells. 



9. Older stage ; lettering as before. First appearance of the limestone rods of the Pluteus. 



10. .Still older stage ; r, tlie limestone rods of the Pluteus have increased in size. 



11. .Slightly older ; the digestive cavity, d, is outlined. 



12. The embryo assumes somewhat the Pluteus outline. 



13. Young Pluteus seen from the dorsal side. ;), general cavity ; /, limestone rods ; a, anal region of the digest- 



ive cavity. 



14-26. OphiothrixfrnrilJh. From .1. Miiller, Ueber die Ophiurenlarven des Adriatisclien Heeres, Berlin (Pt. 5), 

 (1851,) Pis. VI., VII., VIII. Abhand. d. K. Akad. d. Wiss., Berlin, 1852. 



14-17. Younger stages of the Pluteus. m, moutli ; d, digestive cavity. 



18. Pluteus at a stage in which all the arms are developed, although the two long arms have by no means reached 



their full length (see fig. 19). ni, mouth ; o, ffisophagus; d, digestive cavity ; i', intestine ; w, u', lateral 

 disks of water-tubes ; t, lobed water-system. 



19. Pluteus of same, in which the two long arms have reached their full length : lettering as in fig. 18. 



20. The long arms of the Pluteus alone remain (extremities omitted) ; the others have been mostly resorbed ; the 



young ophiuran has assumed a pentagonal outline ; the plates of the abactinal system, the terminal aim 

 plates, and the interbraehial plates, are represented by j/-shaped rods, t', tenninal lobes (tentacles) ; 

 fi, actinal lobes ; fl, second i)air of lobes. 



21. Pentagonal Ophiothrix, seen from the dors.il side : the teeth are seen through the disk. 



22. The same as fig. 21, seen from the actinal side, the arms folded towards the aetinal opening, showing also the 



hooks, the teeth, and the true mouth in the depth of the central star-shaped mouth. 



23. Slightly older, seen in profile : the long arms of figs. 20-23 are still of full length. 



24. A young Ophiothrix after the loss of the two long arras, seen from the dorsal side. 



25. Somewliat older than stage of fig. 23, seen from the actinal side, t', fl, fi, fi, terminal, actinal, and second 



and third pairs of tentacles. 



26. Young Ophiothrix, seen from the abactinal side, with two arm-joints. All traces of the Pluteus have dis- 



appeared. 



27-34. Pluteus bimaculatus. From J. Miiller, ITeber die Ophiurenlarven des Adriatischen Meeres, Pis. IV., V., 



quoted above. 



27. Shows the young ophiuran at the time when the arms of the Pluteus begin to be resorbed and the tentacular 



lobes are arranged in a horseshoe shape round the actinostome ; lettering as in fig. 25. 



28. Somewhat older stage, seen from the actinal side ; the young ophiuran has assumed a pentagonal outline ; the 



tenninal arm plates and the interbraehial plates are well developed ; the actinal tentacles are bent in 

 towards the centre of the actinostome. 



29. Still older stage, also seen from the actinal side, with three pairs of tentacles. 



30. Somewhat older stage, seen from the abactinal side ; the plates of the disk, the centrodorsal, radials, and 



intermediates, are well developed. 



