THE UKVKLOPMENT OF OPHIOTHRIX FKAGILIS. 601 



ciliated baud of llie larva, cil. ad. Adoral ciliated band of I lie larva, coe. 

 Undivided coelomic rudiment, ep. Epineural fold. ep. s. Epineurai sinus or 

 canal, gen. Primitive germ-cells, int. Intestine. l.a.c. Left anlerior 

 coelonv. /. coe. Left coelomic vesicle as yet undivided. I. hi/. Left lijdrocculo. 

 I. p. c. Left posterior ccelom. I'.p'.c'. Ventral lioru of left posterior cceloni. 

 /". //'. c". Dorsal horn of left posterior coclom. mes. Mesenchyme. me&^. 

 Primary mcscncliyme. mes^. Secondary mesenchyme, mp. Primary madre- 

 poric pore. muse. Muscle-fibres, n. c. Nerve-cells of eclodermic origin. 

 u', c'. Nerve-cells of coelomic origin, n.f, Nervc-Mbres. oes. ffisophagus. 

 p. c. Pore-ranal. /)f/. Postero-dorsal arm of larva, ph. Perihaemal space. 

 ph. 1, 2. PerilicEmal rudiment originating between hydrocoele lobes 1 and 2. 

 ph. 2, 3, ph. 3, 4, ph. 4, 5, ph. 5, 1. Rudiments originating between lobes 

 2 and 3, 3 and 4, 4 and 5, and 5 and 1 respectively. pL Postero-lateral arm 

 of larva. /;. o. Post-oral arm of larva, r. a. c. Right anterior coeloni. r. cce. 

 Right coelomic vesicle as yet undivided, r. hi/. Right hydrocele, r.p.c. 

 Rigiit posterior coelom. sk. Larval skeletal rod. sk'. Rudiment of adult 

 skeleton, sp. Buccal spines, or "teeth" of adult. si. Stomach, st.c. 

 Stone-canal, stom. Stomoda^um. vac. Vacuolated ectodeim at posterior end 

 of larva. 



PLATE 3L 



EiG. L — Optical section of blastula, seven hours old, reared from natural 

 fertilisation, x 260. 



Pig. 2. — Optical section of blastula, seven lioiirs old, reared from natural 

 fertilisation, but more advanced in development than that shown in fig. L 

 mes. Primary mesenchyme, x 260. 



Fig. 3. — Optical section of gastrula, twenty-one hours old, reared from natural 

 fertilisation. The vacuolated crest is already strongly developed, x 260. 



Pig. 4. — Ventral view of larva, thirty hours old, reared from natural 

 fertilisation, cil. longitudinal ciliated band, x 260. 



Pig. 5. — Ventral view of larva of same age and origin as that shown in fig. 4, 

 but of more advanced development, x 2G0. 



Pig. 6. — Ventral view of larva, fifty-five hours old, reared from natural 

 feriilisation. r. coe., I. cce. Right and left undivided coelomic vesicles, p. I. 

 Postero-lateral larval arms. sk. Calcified rods supporting the larval arms. 

 X 260. 



Pig. 7. — Ventral view of larva, three days old, reared from natural fertilisa- 

 tion, a. I. Anlero-lateral larval arms. p.o. Post-oral larval arms. X 260. 



Pig. 8. — Dorsal view of rather older larva picked out of Plankton. The 

 larval skeleton has been dissolved out by the preservative, l.a.c, l.p.c. Anterior 

 and posterior halves of the left coelom. r. a. c, r.p.c. Anterior and posterior 

 halves of the right ccelom, still connected, x 260. 



