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. Jledian luufjitudiual section of a larva in Stage "A." 



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

 Fig. 5. Transverse section of a larva iu Stage " B " in a plane halfway between the blastopore and the point where 



the hydrocele is connected with the archenteron. 

 Fig. 6. Larva in Staj;e '• C," seen from the ventral side, the ventral half of the ectoderm, the mesenchyme, and part 



of the u'sophagus removed. 

 Fig. 7. The left half of the ^aine larva. 



Fig. 8. Tran.sverse section tbronj;h Stage "C" in a plane indicated on fig. 6 by the letters o-ft. 

 Fig. 9. The recon.structed internal anatomy of a larva in Stage " D," the ventral ectoderm removed and with it part 



of the a^sopllagus. 

 Fig. 10. An outline drawing of tig. 9, on which are iudicat<d by lines the planes of the sections which follow in 



figures 11, 12, 13, and 11. 

 Fig. 11. Longitudinal section taken through a, larva in Stage " D " in the plane indicated on fig. 10 by the lino m-n. 

 Figs. 12, 13, and fig. 11 of Plate II. Transverse sections taken through Stage "D" in planes indicated on fig. 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 first 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 Hues 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-8, t-u, and v-w. 

 Fig. 31. Reconstruction of the an.atomy of a larva in Stage "G." In this case as In all the other reconstrnctions 



the ventral surface is up and the ventral ectoderm rcuioved. 

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

 Fig. 33. Transverse section of Stage "H," taken through the regiou of the stone canal. 

 Fig. 34. Transverse section of a larva iu Stage "H," taken through the stomach. 



Fl.\te 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 seotious, which have been drawn to show 



the anatomy of a larva in Stage " F." 

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



by the lines a-h, c-d, m-ii, and x-y. 

 Figs. 28, 29, aud 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. 



