DEVELOPMENT OF CUCUMARIA ECHINATA. 



385 



ventral radial canal and the Polian vesicle are given out from the end 

 of the ventral limb of the still open hydrocœle (Textfig. F, Pl. V, figs. 

 4 and 6, for and pv). Thus it is clear that the ring closes at the left 

 dorsal interradius. The appearance of the Polian vesicle may be still 

 later. It lies at first very near to the ventral radius, but afterwards 

 assumes its final position in the left dorsal interradius. 



The gut, whose internal lumen was almost obliterated in the 

 dipleurula stage, has been rapidly growing during the transformation 

 of the larva into the doliolaria. It now extends as far forwards as the 

 hydroccele-ring, has a distinct lumen, is flattened in the frontal plane, 

 and has a thick entodermal wall. The posterior end of the gut is still 

 solid, and the anal depression is present at the posterior end of the 

 body. 



The above changes are gone through very rapidly, and now the 

 doliolaria is completed. The stage lasts till the end of the fourth day 

 of development, or it may be prolonged still further, to the eighth day. 

 While no marked change is visible from the exterior, the internal 

 organs are gradually growing further during the rest of this stage. 



At about the middle of the primary stone-canal there is formed a 

 dilated part, the madreporic vesicle (Textfigs. F and G, Pl. V, figs. 4 

 and 6, mp), with its wall thickened on the side of the body-axis. 

 The distal portion of the canal, which is now to be called the pore- 

 canal (pc), is very fine, and opens externally by a minute dorsal pore 

 (dp). The mid ventral radial canal becomes finer, and the first pair of 

 pedicels are seen as cylindrical projections above the niveau of the 

 body-surface. The tentacles have extruded through the widened 

 stomodeal orifice, with minute papilla; on the tip, and subsequently 

 giving out some knob-like branches. Of the paired radial canals the 

 dorsal pair elongate faster than the ventral, and reach a little beyond 

 the dorsal pore (Textfig. G, Idr and rdr). All the radial canals are 

 now accompanied by the radial nerves and epineural canals. The 

 enterocœle now widens and the mesenchyme filling the blastocœle 



