TllK DEVEI.OPMENT OE ECHIXOCAKD[U>[ COK’DATU:\r. 485 
Calcareous centre for the formation of the larval skeleton, cl. sp. 
Clavate spine of the adult, cce. Ccelom sac. d. a. Dorsal arch. dent. 
Spine possibly homologous with the tooth of a regular Echinoid. h. r. 
Horizontal rod. liy. Hydroccele. liy. 1. Lobe of hydrocoele. int. 
Intestine. 1. a. Vestiges of larval arms, l.a.c. Left anterior coelom. 
1. h. Larval brain. 1. p. c. Left posterior coelom, c’s. (Esophagus, o. 1. 
Oral lobe of the larva, p. d. a. Postero-dorsal arm. d. r. Postero- 
dorsal rod. p. 1. a. Postero-lateral arm. p. 1. r. Postero-lateral rod. 
p. mes. Primitive mesenchyme, p. o. a. Post-oral arm. p. o. r. 
Post-oral rod. pr. o. a. Prse-oral arm. r.a.c. Right anterior coelom. 
r. p. c. Right posterior coelom. 7\ r. Recurrent rod. s.mes. Secon- 
dary mesenchyme, spli. Sphaeridium. st. Stomach, stom. Stomodaeum. 
ten. Primary tentacle of the water-vascular system, vac. Vacuity in the 
wel) connecting the two post-oral arms. 
Fig. 1. — Blastula 15 hours old. X 200. ap. Apical plate carrying 
specially long cilia, p. mes. Primitive mesenchyme. 
Fig, 2. — Gastrula about 32 hours old. X 200. calc. Centre of cal- 
cification for the larval skeleton, s. mes. Secondary mesenchyme. 
Fig. 3. — Larva 2 days old viewed from the ventral surface. X 200. 
a. Larval arms. ah. mes. Mass of mesenchyme in which the skeleton 
of the aboral spike is formed, b. r. Body-rod of skeleton, cce. LTn- 
paired rudiment of the coelom, h. r. Horizontal rod of skeleton, p. o. a. 
Rudimentary post-oral arm. p. o. r. Post-oral rod. 
Fig. 4. — Larva of same age as last, but more advanced in develop- 
ment, viewed from the dorsal surface. X 125. ah. cede. Calcareous 
centre for the formation of the aboral rod. a. 1. r. Antero-lateral rod. 
cce. Coelomic sac. r. r. Recurrent rod. stom. Stomoda3um. 
Fig. 5. — Larva 3 days old viewed from the dorsal surface. X 125. 
ad. r. Aboral rod. a. la. Antero-lateral arm. ces. (Esophagus. st. 
Stomach. 
Fig. 6. — Larva 8 days old viewed from the dorsal surface, x 125. 
ad. cil. Adoral ciliated band. d. a. Dorsal arch. hy. Hydrocoele. 
p. d. a. Postero-dorsal arm. p. d. r. Postero-dorsal rod. pr. o. a. Pra?- 
oral arm. 
Fig. 7.— Larva 9 days old viewed from the ventral surface. x 125. 
p. 1. r. Postero-lateral rod. 
Fig. 8. — Larva 9 days old a little more advanced in development 
than that represented in fig. 7 viewed from the dorsal surface, x 125. 
ah. cil. Aboral crest of cilia, am. Amniotic cavity, hy.l. Lobes of the 
hydrocoele. p. 1. a. Postero-lateral arm. 
Fig. 9. — Larva 16 days old viewed from the dorsal surface, x 10(t 
a. d. a. Antero-dorsal arm. 
