Echinoderm Larvce with Two Water- Vascular Systems. 347 



Fig. 3. — Larva 18 days old viewed from the dorsal aspect, showing the first appearance 

 of a rudiment of the right hydi-occele. This larva was transferred to 

 hypertonic water on the third day and remained in it. Drawn from life 

 from the cultures of 1914. Magnification 80 diameters. 



Fig. 4. — Larva about 32 days old, from the cultures of 1917, reared entirely in normal 

 water, and viewed from the dorsal aspect. Preservation 40-per-cent. foiTualin. 

 Magnification 80 diameters. This larva and that represented in the two 

 following figures give an idea of the stage of development attained by the 

 larvse in the control plunger-jar when the experiments of 1917 were closed ; 

 m.'p., madreporic pore ; st.c, stone-canal, the reference line points to the 

 opening of this canal into the axial sinus (ax.). 



Plate 5. 



Fig. 5. — Larva entirely similar to that represented in fig. 4, viewed from the left side ; 



mad., madreporic plate ; Ech., Echinus-rudiment. 

 Fig. 6. — The 'same larva as that represented in fig. 5, viewed from the right side ; calc, 



calcareous plate within the loop of the ciliated band midway between the 



two pedicellari?e. 



Fig. 7. — Larva 40 days old, which had been exposed to hypertonic sea-water, viewed from 

 the dorsal aspect. Drawn from life from the cultures of ] 916. Magnification 

 about 50 diameters. 



Xote l.a.c, the left anterior coelom, i.e., the axial sinus into which the 

 stone-canal (sf.c.) opens ; r.a..c., the right anterior coelom, expanded into 

 a supplementary axial sinus ; r.hy., the right hydrocoele. 



Figs. 8-11, inclusive, represent larvae about 32 days old, from the cultures of 1917, 

 which have been exposed to the action of hypertonic water and have been subsequently 

 transferred to normal sea-water in a plunger jar. All have been preserved in 40-per- 

 cent, formalin, and all are viewed from the dorsal aspect under a magnification of 

 65 diameters. 



Fig. 8. — Larva with two hydrocceles of almost equal size and no pedicellariae. 



Note ab.t., aberrant tentacle, a lobe of the right hydroccele projecting into 

 the blastoccele ; m.p., m'.p'., the two madreporic pores. 



Fig. 9. — Larva with the right hydroccele smaller than the left but devoid of pedicellariae. 



Note the single axial sinus into which the two stone-canals (-s^c, st'.c'.) 

 open ; m.p., the single madreporic pore. 



Plate 6. 



Larva with two hydrocceles of nearly equal size but with a single pedicellaria 

 on the right side {ped.) ; m.p., the single madreporic pore ; v.c.ep., the ventral 

 ciliated epaulette seeb through the transparent body-wall. 

 Larva with two hydrocceles, of which the right is small and without lobes. 

 No amniotic cavity is developed on the right side ; ax., left axial sinus ; 

 a.v.', right axial sinus ; m.p., m'.p'., the two madreporic pores. Two 

 pedicellaria are developed {ped.). 



Figs. 12-14 represent larvte from the cultures of 1917 which had been starved for 

 the first week and then generously fed. They were about 32 days old, and were 

 preserved in 40-per-cent. formalin. Magnification 65 diameters. 



