604 E. W. MACBIMDE. 



PLATE 34. 



Fig. 32. — Lonjjitudinal frontal section of a larva picked out of Plankton 

 Estimated age four and a half days. I. p. c. Outgrowth from the left ccelomic 

 sac, which will be cut off as the left posterior ccelom. al. Antero-lateral arm. 

 X 260. 



EiG. 33. — Longitudinal frontal section of a larva four and a half days old. 

 Natural fertilisation, l.p.c. The rudiment of the left posterior coeloni. It 

 has now assumed a rounded shape. X 2C0. 



EiG. 3i. — Longitudinal frontal section of a larva picked out of Plankton. 

 Estimated age five days, r.p.c. Outgrowth from right ccelomic sac, which will 

 he cut off as the right posterior ccelom. x 260. 



Fig. 35. — Transverse section of a larva three days old. Natural fertilisation. 

 mp. Ectodermic invagination to form the madreporic pore, x 500. 



Fig. 30. — Transverse section of a larva three days old, more advanced in 

 development than that shown in the preceding figure. Natural fertilisation. 

 mp. Ectodermic invagination to form madreporic pore ; this has now fused 

 witii an outgrowth from the left ccelomic sac. st. Stomach. X 500. 



Figs. 37 a and b. — Two longitudinal frontal sections through a larva picked 

 out of Plankton of estimated age fourteen to sixteen days. Fig. S/^"/ is the 

 more dorsal section passing tiirough the oesophagus. Fig. 2>7b is a more 

 ventral section passing through the stomach alone, r. hi/., I. hi/. Rudiments of 

 tiie right and left liydrocceles respectively, pd. Postero-dorsal arms : the 

 l.Hst to appear. X 260. 



Figs. 3S a and b. — Two longitudinal frontal sections through a larva some- 

 what older than that represented in fig. 37, picked out of Plankton. Fig. 38rt 

 passes through the left hydrocoele, fig. 38 b through the right hydroccele. The 

 latter is the more dorsal section. X 260. 



Fig. 39. — Longitudinal frontal section through a larva picked out of 

 Plankton, still older than that shown in fig. 38. The left liydioccele is just 

 beginning to be lobed ; 1—5. Its lobes, x 260. 



Fig. 40.— Longitudinal frontal section tiirough a larva picked out of 

 Plankton older than that shown in fig. 39. The lobes of the left hydroccele 

 are finger-like, r.a.c, l.a.c. lUght and left anterior cceloms, just grazed. 

 cil. Longitudinal ciliated band. cil. ad. Adoral ciliated band, x 200. 



Fig. 41. — Longitudinal median sagittal section through a larva picked out 

 of Plankton, about the same age as that shown in fig. 40. x 260. 



PLATE 35. 

 Figs. 42 a and b. — Two longitudinal sagittal sections through a larva 

 picked out of Plankton, of same age as that shown in the preceding figure. 

 Fig. 42 a is more to the left of the middle line than fig. 42 b. Fig. 42 a sliows 



