247 



vegetal axis. The gastrula forms by the end of the first day of development. 

 It is highly transparent, slightly flattened at the vegetal pole, with short cilia 

 completely covering its surface. At the end of the second day the gastrula 

 transforms into a dipleurula. This stage may last as long as two days. There- 

 after the next stage, the auricularia, sets in, which is the longest stage in the 

 development of the trepang. 



Auricularia 



The larva is transparent. In each comer of the anal lobe an irregularly 

 shaped calcite ossicle forms, which is much larger in the left comer. Under 

 the ciliary band, at the ends of the lobes, there are five pairs of round cavities, 

 the elastic spheres. Mortensen (1937) mentioned similar stmctures while 

 describing the larvae of Holothuria arenicola and H. scabra. These cavities 

 are possibly the organizing centers from which formation of the ciliated bands 

 of the doliolaria originates. After their appearance, the oral and anal lobes 

 reduce and the ciliated band on them disappears. In the late auricularia the 

 lateral lobes are smooth. Before transition to the doliolaria, the length of the 

 auricularia is about 400 |im (Figures 190, 191). 



Doliolaria 



This stage continues for two days. It is characterized by a barrel-shaped 

 larva in either a ventral or dorsal view. In a lateral view it is clearly apparent 

 that the depression in the region of the mouth persists. The ftilly developed 

 doliolaria has five ciliated bands: anterolateral, preoral, oral, preanal, and 

 postanal. The doliolaria is less transparent than the auricularia. The calcite 

 ossicles are large and lie under the postanal band along the comers of the 

 doliolaria. The anterior and under the anterolateral band thickens. The length 

 of the larva at this stage is 400 |J.m (Figs. 192, 193). 



Prepentactula I 



The larva is elongate. The round cavities are ahnost indistinguishable but 

 the ciliated bands are completely retained. The calcite granules underlying 

 the postanal band gather at this stage in the middle of the posterior end of 

 the larva. The five primary tentacles are distinctly visible through the integu- 

 ment; they are sometimes extmded and again retracted into the vestibular 

 cavity. Further, the perioral calcite ring and the madreporite are distinguish- 

 able. Below the calcite ring the unpaired ambulacral podium lies along the 

 stomach. The length of the larva at this stage is about 400 jim (Figure 194). 



