348 Echinoderm LarvcB with Tivo Water- Vascular Systems. 



Pig. 12. — Larva viewed from the dorsal aspect : l.sp., the left group of spines within the 

 loop of the ciliated band ; r.sp., the right group of spines within the loop 

 of the ciliated band ; I'.sp'., spine on the left side outside the loop of the 

 ciliated band ; r'.sp'., spine on the right side outside the loop of the ciliated 

 band. 



Fig. 13. — Larva viewed from the left side : m.sp., madreporic spine. 



Fig. 14. — The same larva as that shown in fig. 13, viewed from the right side. 



Figs. 15-23 represent transverse sections through larvae presei'ved in Bouin's fluid. 

 All are from the cultures of 1917, aud are about 32 days old, except where otherwise 

 stated. 



Plate 7. 



Fig. 15. — Section of the normal larva showing the mutual relations of axial sinus, madre- 

 poric vesicle, pore-canal and madreporic pore. Magnification 120 diameters. 



Fig. 16. — Section of normal larva to show the mutual relations of the Echinus-rudiment 

 and the pedicellarise. Magnification 120 diameters. 



Fig. 17. — Section of larva with right and left pore-canals {p.c. and p'.c'.). In another 

 section from the same series these canals are seen to unite, to open by a single 

 median pore to the exterior. Magnification 105 diameters. 



Figs. 18 and 19 represent two sections of the same larva. The larva possessed two 

 well-developed hydrocoeles and one pedicellaria. Magnification 105 diameters. 



Plate 8. 



Fig. 18 shows the single pore-canal, the single median axial sinus (representing the 

 combination of right and left axial sinuses), and the relation of this com- 

 pound axial sinus to the madreporic vesicle. 



Fig. 19 shows the two hydrocoeles : d.s., dental sac of the left side ; d'.s'., dental sac 

 of the right side. 



Plate 9. 



Fig. 20. — Section of a larva with two hydrocoeles and one pore-canal. The median 



axial sinus has caused the complete suppression of the madreporic vesicle. 



Magnification 120 diameters. 

 Fig. 21. — Section of a larva 40 days old, from the cultures of 1916, which has two 



hydrocoeles. Two madreporic pores {m.p. and m'.p'.) are present, but both 



belong to the left side. Magnification 120 diameters. 



Figs. 22 and 23 represent two sections of the same larva. This larva belongs to the 

 cultures of 1917, and is one of those which was starved and then generously fed. 

 Magnification 120 diameters. 



Plate 10. 



Fig. 22 shows the median madreporic vesicle (m.v.). 



Fig. 23 shows the groups of spines which replace the Echinus-rudiment and the 

 pedicellarise. 



