ON THE EMBRYOLOGY OF ECHINODERMS, 29 



croached somewhat on the anal extremily. The spines are quite prominent ; the vihratile epaulettes 

 have acquired a very large size ; two very prominent spots, s', s', in the arms, e', c"; / is perhaps some- 

 thing homologous to the brachiolarian arms of Brachiolaria. 



Fig. 21. View of fig. 20, corresponding to figs. 10, 13, and 18. 



Fig. 22. Profile view of fig. 20. 



Fig. 23. The young Echinus has materially encroached on the Pluteus, and resorption has been carried on 

 to such an extent that there is very little left of the Pluteus except the arms, which have lost their 

 mobihty. 



Fig. 24. A young Echinus immediately after the resorption of the Pluteus, seen from the abactinal side. 



Fig. 25. Young Sea-urchin somewhat more advanced than that of fig. 24, seen from the abactinal side. The 

 opening of th"fe anus (a, fig. 28) cannot be traced in these younger specimens, though it is very apparent 

 in somewhat more advanced ones. 



Fig. 26. The same as fig. 25, seen from the actinal side ; the tentacles have become more slender ; the spines 

 more numerous than in younger stages. The actinal membrane is well developed. 



Fig. 27. The same as figs. 25 and 26, stripped of its spines, seen from the actinal side. 



Fig. 28 is a young Toxopneustes measuring one fifteenth of an inch in diameter, including the length of the 

 spines, in which pedicellaria have developed, and the spines have assumed the general appearance of 

 those of the adult. 



OPHIOPHOLIS BELLIS Lyman. 

 Figs. 29, 30. 

 Fig. 29. Abactinal view of a young Ophiopholis, to show the arrangement of the plates of the disk. 

 Fig. 30. A somewhat younger Ophiopholis than fig. 29, seen from the actinal side. 



AJVIPHIURA SQUAMATA Sabs. 



Figs. 31-33. 



Fi". 31. An adult Pluteus of Amphiura, in which the water-tubes are plainly seen. The figure is taken on 

 the side of the madreporic body ; one of the long arms is cut olT. In this figure v' is the anal, and v the 

 oral vibratile chord. 



Fig. 32. The arms of the Pluteus are almost entirely resorbed, except the two long arms, e', which are still 

 intact, as in fig. 31 ; they are here cut off for want of space. The Ophiuran is seen from the abactinal 

 side, to show the arrangement of the arm-plates, i/, y'. 



Fi"^. 33. The same as fig. 32, seen from the actinal side. The mouth is still very large ; the tentacles, t, V, 

 mere loops. 



ASTERACANTHION FLACCIDA Ag. 

 Fi". 34. Mode of carrying the embryos in Astcracanthion Jlaccida. 



