28 ON THE EMBRYOLOGY OF ECHINODERMS. 



strictions ; the pouch, o, formed at the blind extremity, has sent off two diverticula, w, w' ; the first 

 appearance of the water-tubes. End of fourth day. 



Fig. 5. Profile view of an embryo somewhat more advanced than fig. 4, at the beginning of the fifth day ; 

 the intestine, c, stomach, d, and oesophagus, o, are well separated ; the water-tube, w', also stands out 

 prominently from the extremity of the cesophagus, o ; the anal part of vibratile chord, v, bulges out 

 considerably. 



Fig. 6 is a view, from the mouth-side, of the stage the embryo has attained at the end of the fifth day. The 

 water-tubes, w, w', have different degi-ees of development, and are less closely connected with the oesoph- 

 agus ; the rods have sent out processes at points where the new arms are to be developed, at v. 



Fig. 7 is a view of fig. 6 from the anal extremity, to show the changes of form since fig. 2, and the position of 

 the vibratile chord. 



Fig. 8 is a profile view of fig. 6 ; the mouth, 7n, is not yet opened. 



Fig. 9. A profile view of an embryo taken at the beginning of the seventh day ; the mouth, m, is opened ; 

 the water-tube, w', reaches nearly the dorsal surface. The currents which previously to this stage had 

 carried the food through the only opening, a, as far as o, and then were reversed to eject the digested 

 matter, now come in through the mouth, m, pass through the oesophagus, rotate about in the stomach, d, 

 and pass out through the first-formed opening, the anus a, which is hereafter used only to eject the food. 



Fig. 10 represents a Pluteus at the beginning of the eighth day, seen in such a manner as to show the whole 

 of the vibratile chord, when looking on the side where the mouth and anus open. 



Fig. 11 is a profile view of an embryo slightly more advanced than the one represented in fig. 10. 



Fig. 12. The same as fig. 11, seen from the mouth side. 



Fig. 13. Somewhat more advanced than fig. 12, seen so as to show the changes the vibratile chord has under- 

 gone since the stage represented in fig. 10. 



Fig. 14. An embryo during the tenth day, seen in profile ; shows the position of the arms e'" and e'". 



Fig. 15. The same as fig. 14, seen from the mouth side ; the arms, e', have been greatly developed ; the 

 differentiation of the intestine, c, the stomach, d, and oesophagus, o, is quite complete. First appearance 

 of the vibratile epaulettes, v". The water-tubes have not united ; they have not greatly increased in size. 



Fig. 16. Profile view during the twenty-third day; the arm, e", has made its appearance, and is already 

 quite prominent ; a still greater development of the vibratile epaulettes is perceived. 



Fig. 17. The same as fig. 16, seen from above, to show the relation of the different rods to each other, as 

 well as the first appearance of the tentacular loop, t. 



Fig. 18. View of fig. 17, corresponding to the less advanced stages of figs. 10 and 13, showing the connec- 

 tion of the different parts of the vibratile chord. 



Fig. 19. A much more advanced stage than fig. 17, seen from above ; found swimming freely on the surface 

 of the water. Tlie rods extending into the arms are made up of three sets of rods united by short trans- 

 verse bars ; additional tentacular loops have been formed. The water-tubes have greatly increased in 

 size, and appear to have united. 



Fig. 20. An adult Pluteus of Toxopneustes drobachiensis, in which the young Sea-urchin has already eii- 



