STUDIES ON THP] DEVELOPMENT OK ECHINOIDEA. 
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(lays old. cos. CcBlomic sac just separating from archenteron. b. r. 
Body-rod. h. r. Horizontal rod, and p. o. post-oral rod as three branches 
of trifid calcareoiis star, the laidiment of the larval skeleton. 
Fig. 4. — Larva of Echinus esciilentus thi-ee days old viewed from 
the dorsal side. a. I. antero-lateral rod of skeleton, cos. Coelomic sacs. 
oes. Endodermal part of oesophagus, stom. Stomodseum. p. o. Post- 
oral rod of skeleton (notice the supplementary rod on right side). 
Fig. 5. — Larva of E c h in u s e s c xi 1 e n t u s four days old viewed from the 
dorsal surface, ad. Thickening of epithelium carrying adoral ciliated 
band. dil. Dilator muscles of coelomic sac and of oesophagus attaching 
these structiu'es to the antero-lateral rod of the larval skeleton, mad. 
Madreporic pore. 
Fig. 6. — Larva of Echinus esculentus six days old viewed from 
the dorsal surface, cr. Inbent crook at aboral end of body-rod. p. d. 
Accumulation of mesenchyme cells, the formative tissue of skeleton 
of postero-dorsal arm. p. r. o. Rudiment of prae-oral arm. r. r. Vestige of 
recuiTent rod of skeleton. 
Fig. 7. — Blastula of Echinocardium cordatum twenty-four 
hours after fertilisation, mes. Primary mesenchyme being formed. 
Fig. 8. — Gastrula of Echinocardium cordatum one and a half 
days old. calc. Rudiment of skeleton, cil. Anterior tuft of long cilia. 
mes. Secondary mesenchyme being budded from the apex of the archen- 
teron. 
Fig. 9. — Larva of Echinocardium cordatum two days old viewed 
from the doi'sal surface, ah. Calcai’eous star, the rudiment of the skele- 
ton of the aboral spike embedded in a mass of formative mesenchyme. 
p. o. Latticed skeleton of post-oral arm. ces. Endodermal oesophagus 
which is in contact with, but which has not yet opened into, stom. 
the stomodffium. r. r. Recurrent rod of the skeleton. 
Fig 10. — Larva of Echinocardium cordatitm three days old 
viewed from the dorsal surface. The antero-lateral arms have grown 
out and the stomodajum has opened into the oesophagus. The aboral 
spike and its skeleton are fully formed, cil. Posterior Lift of cilia 
at the apex of the aboral spike. 
Fig. 11. — ^Larva of Echinocardium cordatum four days old, 
viewed from the dorsal surface. 1. p. c. Backward growth of the coelom 
on the left side of the stomach, which wiU be cut off as the left posterior 
coelom, r.p.c. Similar growth of the coelom on the right side of the 
stomach which will be cut oft’ as the right posterior coelom. mad. 
Left madreporic pore, wood.' Right maoli-eporic pore. p.d. Rudiment 
of the skeleton of the postero-dorsal arm. 
