100 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



PLATE I. 



FIG. 1. Larva in Stage "A," seen from the right side, the right half of the ectoderm removed and the mesenchyme 



omitted. 



FIG. 2. The same larva, seen from the ventral side as a transparent object. 

 FIG. 3. Median longitudinal section of a larva in Stage "A." 



FIG. 4. A larva in Stage " B," viewed from the ventral side as a transparent object. 

 FIG. 5. Transverse section of a larva in Stage "B" in a plane halfway between the blastoporo and the point where 



the hydroeele is connected with the archentermi. 

 FIG. 6. Larva in Stage " C," seen from the ventral side, the ventral half of the ectoderm, the mesenchymo, and part 



of the oesophagus removed. 

 FIG. 7. The left half of the same larva. 



FIG. 8. Transverse section through Stage "C" in a plane indicated on fig. 6 by the letters a-6. 

 FIG. 9. The reconstructed internal anatomy of a larva in Stage " D," the veutral ectoderm removed and with it part 



of the oesophagus. 

 FIG. 10. An outline drawing of fig. 9, on which are indicated by lines the planes of the sections which follow in 



figures 11, 12, 13, and 14. 



FIG. 11. Longitudinal section taken through a larva in Stage ' : D'' in the plane indicated on tig. 10 by the line m-n. 

 FIGS. 12, 13, and fig. 14 of Plate II. Transverse sections taken through Stage "D" in planes indicated on lig. 10 by 



the lines a-b, c-d, and e-f. 



PLATE II. 



FIG. 15. Ventral view of a larva in Stage "E," to show ciliated bands and iirst appearance of the arm rudiments. 



FIG. 16. Dorsal view of Stage " E,'' showing the ciliated bands. 



FIG. 17. A reconstruction of the anatomy of a larva in stage "E," the ventral ectoderm removed. 



FIG. 18. An outline drawing of fig. 17, on which are indicated by lines the planes of the sections shown in figs. 19, 



20, and 21. 

 FIGS. 19, 20, and 21. Transverse sections taken through larva in Stage " E " in planes indicated on fig. 18 by the 



lines r-s, t-ii, and r-ic. 

 FIG. 31. Reconstruction of the anatomy of a larva in Stage " G." In this case as in all the other reconstructions 



the ventral surface is up and the ventral ectoderm removed. 



FIG. 32. An outline drawing of the water vascular system of a larva in Stage "G," seen from the ventral surface. 

 FIG. 33. Transverse section of Stage "H," taken through the region of the stone canal. 

 FIG. 34. Transverse section of a larva in Stage "H," taken through the stomach. 



PLATE III. 



FIG. 22. Ventral view of the fully developed larva before metamorphosis has begun. Stage "F." 



FIG. 23. Outline drawing of fig. 22. The lines indicate the planes of the sections, which have been drawn to show 



the anatomy of a larva in Stage "F." 

 FIGS. 24, 25, 26, and 27. Longitudinal sections of a larva in Stage " F," the planes of which are indicated on fig. 23 



by the lines a-b, c-d, m-n, and jr-y. 

 FiGS. 28, 29, and 30. Transverse sections of a larva in Stage "F,'' the planes of which are indicated on fig. 23 by 



the lines c-o, d-o and e-o. 



