THE DEVELOPMENT OE ASTEEINA GIBBOSA. 285 



Fig. 26. — Section 6f larva rather older than Stage B, to show the forma, 

 tion of the madreporio pore. pc. Pore-canal ending blindly in contact with 

 the ectoderm. /. siom. Larval stomach, mp. Thickening of ectoderm where 

 the primary madreporio pore will be formed. 



Figs. 37 — 29. — Three sections of a larva slightly older than the preceding. 

 Fig. 27 shows that on the left side the coelom is divided into an anterior 

 coelom a, and a left posterior coelom Ipc. Fig. 28 shows that this division 

 only extends about halfway to the ventral side. Fig. 29 shows that the 

 separation of the coelom from the archenteron commences ventrally, since here 

 the coelom is shut off from the gut. tr. First trabecula. 



Fig. 30. — Section of a larva rather older than that shown in Figs. 27 — 29. 

 Ice. Larval oesophagus, ir. Trabeculse cords of cell spanning the left pos- 

 terior coelom. 



Fig. 31. — Sagittal section of larva about Stage B, to show the formation of 

 the larval oesophagus. It is clearly seen that this is a stomodseum which has 

 not as yet joined the gut. 



Figs. 32 — 34. — Three sections of a larva younger than Stage C. The 

 segmentation of the coelom on the left side is complete; on the right side it 

 has begun dorsally (Fig. 32). The left water-vascular rudiment or hydroccele 

 (My.) has appeared as an outgrowth of the anterior coelom, its lobes numbered 

 as in PI. 18. Fig. 32 shows Nos. 1 and 2 ; Fig. 33, No. 3 ; and Fig. 34, Nos. 

 i and 5. 



Fig. 3-5.— Section of a larva of Stage C. The first trace of the right hydro- 

 coele (rh^.) has appeared. 



Fig. 36.— Section of a slightly older larva than preceding. The develop, 

 ment of the right hydroccele is more advanced. 



Figs. 37— 41.— Five sections of a larva of Stage D, or slightly younger. 

 In Fig. 37 we see a section of the pore-canal (pc.) and the origin of the rudi- 

 ment of the oral coelom {or. e.). In Fig. 38 the fully developed form of the 

 right hydroccele (rig.) is shown. In Figs. 39 and 40 we see the left posterior 

 coelom extending obliquely beneath the right posterior ccelom {rpe.) ; this is 

 the right ventral horn {I'p'c'.) of the left posterior ccelom. In Fig. 41 we 

 see it opening into the anterior ccelom. 



PLATE XIII. 



The same remarks apply to this as to Plate 19, but in addition it is to be 

 remarked that the epithelium of the pore-canal and of the stone-canal is 

 distinguished by a cross-striation. 



Figs. 43 and 43.— Two sections of a larva rather younger than Stage D. 

 sic. Rudiment of the stone-canal. Fig. 42 shows the septum between the 

 anterior coelom and the left posterior coelom broken down dorsally; and Fig. 

 43 shows that the septum between the anterior coelom and the right posterior 

 coelom is still incomplete ventrally. 



VOL. 6. ^ 



