284 E. W. MACBRIDE. 



Pig. 9. — Anterior view of the same larva of six days. 



Fig. 10. — Left view of fully developed larva of seven days. Stage D. 



Fig. 11. — The same drawn in the position it assumes in life. 



Fig. 12. — Left view of larva in which metamorphosis has commenced, and 

 which has fixed itself. The Arabic figures denote the primary lobes of the 

 water-vascular system or hydroccele, the Roman figures the rudiments of the 

 arms. The larval organ has disappeared. Stage E. 



Fig. 13. — Right view of the same larva. 



Fig. 14. — Ventral view of larva of about nine days. The arm rudiments 

 form a nearly complete circle. The lobes of the water- vascular system have 

 developed two pairs of accessory lobes each. Stage F. 



Fig. 15. — Right view of the same larva of about nine days. 



Fig. 16. — Left view of the same larva of about nine days. 



Fig. 17. — Oral view of just metamorphosed star-fish about ten days old. 

 amb. Ambulacral ossicles. Stage G. 



Fig. 18. — Aboral view of another specimen of the same age. C. Central 

 plate. B. Basal. T. Terminal. The curve of the arm rudiments has become 

 a circle, No. V coming to be apposed to the lobe No. 1 of the water-vascular 

 system, mp. Madreporic pore. Stage G. 



Fig. 19. — Aboral view of a young star-fish sixteen days old. Notice the 

 anus, the additional calcareous plates, and the spines. 



PLATE XIL 



All the sections represented in this plate are magnified 80 diameters, and, 

 except where otherwise stated, they have been cut parallel to the "larval 

 plane," i.e. they are horizontal longitudinal sections. Where several sections 

 from the same series are figured, the most dorsal is in every case put first. 

 The darkest shade represents the epithelium of the gut ; the intermediate 

 shade represents ectodermic and coelomic epithelium, including the lining of 

 the derivatives of the coelom ; the lightest shade represents the cavity of the 

 blastocoele with all its contained structures, jelly, fibres, cells, &c., and also 

 the muscular tails of the epithelial cells lining the water-vascular system. 



Figs. 20 and 21. — Two sections of a gastrula a little older than Stage A. 

 No mesenchyme has as yet appeared. 



Fig. 22. — Sagittal section of a gastrula about the same stage as Fig. 3. 

 mes. Mesenchyme cells. 



Fig. 23. — Section of an embryo older than that shown in Fig. 3. It shows 

 the differentiation of the archenteron into gut and coelom. 



Figs. 24 and 25. — Two sections of an embryo somewhat younger than Stage 

 B, and still enclosed in the vitelline membrane. The coelom has grown back 

 at each side of the gut, forming two posterior lobes, Ipc, rpc. Fig. 25 

 shows, however, that these lobes do not as yet extend ventral to the gut. 



