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 I l: inning of the fifth day; 

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



a^us, o: the anal part of vibratile chord 



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, < have different degrees of development, and are less closely connect* <1 with the ocsoph- 



arms 



the vibratile chord. 



and 



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



Fig. 9. A profile view of an embryo taken at the beginning of the I venth 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 wen rovers 1 to eject the dig 



irested 



rotate 



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



advanced 



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 vibnuil, chord has under- 



gone since the stage represented in fi". 10. 



anus 



Fig. 15. The same as flg. H, seen from the mouth side , the arms, *, have been greatly developed ; the 



frTn , ' °' ' he St0maeh ' * ^ -"* b '8-' °> " ^ ~°l** Fir,, , PP S mce 



The water-tubes have not united ; they have not greatly 



Fig. 16. Profile view during the twenty-third dav • thp arm -« 1, j • . , , 



. , .„ * iru aa 7 » tn e arm, e", has made its appearance, and is already 



quite prominent; a still tranter <W in™«™ + „*•*!.- •• .... rr ^ 



Fig. 17. 



epaulett 



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



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



^ til JZ^' , 17 ' C0 Tr di " g '° ^ ' eSS adVanCed S ^ 8 of «*• >« •»* 13, showing the conneo 



tion of the different parts of the vibratile chord. 



Fig. 19 



above 



arms 



o 



freely on the surface 



united 



greatly 



An 



