237 



constitutes one-half to three- fourths the length of the auricularia (Kume and 

 Dan, 1968). Retaining contact with the external environment, the oral cavity 

 transforms into a vestibule (vestibulum), homologous with the amniotic sac 

 of the late pluteus of sea urchins. In the vestibule the integument of the 

 former oral cavity surrounds the coelomic diverticulae that shifted here. In the 

 late doliolaria the vestibule is shifted to the anterior end of the larva and 

 formation of the primary tentacles commences in it. 



The hydropore closes and the pore canal turns to the ventral side and 

 transforms into the stone canal, at the end of which the madreporite forms. 

 In Stichopus japonicus the madreporite has already developed in the auricularia 

 stage (Kume and Dan, 1968). In this sea cucumber, at the place of appearance 

 of the fixture medioventral radial ambulacral canal, a small protrusion appears 

 on the ring canal, which initiates the first unpaired ambulacral podium. This 

 podium remains inside the body for some time. In other sea cucumbers the 

 medioventral radial canal elongates and passes along the stomach to the 

 posterior end of the body. Here it bifiarcates and a pair of coelomic rudiments 

 of the ambulacral podia forms on its ends (Kume and Dan, 1968). 



In some sea cucumber species it is possible to identify one more stage, 

 the prepentactula, distinguished by a wide opening of the vestibule from 

 which primary tentacles may protrude. The ciliated bands are retained. In 

 Stichopus japonicus ' at this time, the ambulacral podium shifts to the outside 

 of the body and the larva uses it for attachment to the substrate. The elastic 

 spheres disappear completely. Radial and interradial spicules arise from the 

 perioral calcareous ring. 



Pentactula 



At this stage, the larva has five well-developed primary tentacles; it 

 retains the ciliated bands and skeletal elements formed earlier. Some species 

 have a pentactula stage and, in addition, one or several ambulacral podia 

 (Figure 186). It is in this stage that the definitive organ systems develop. 



Skeleton : Besides the skeletal rods already formed in the auricularia and 

 the perioral calcareous ring, new spicules appear. These are situated along the 

 interradii and constitute the rudiments of intradermal plates and rods. The 

 newly formed spicules are diverse in shape. In some sea cucumber species the 

 skeleton of the pentactula is better developed than that of the definitive 

 individual. 



Digestive system : The oral opening, surrounded by primary tentacles, 

 opens and leads into the esophagus. Primary, and then secondary, tentacles 

 serve as the trapping apparatus substituting for the trapping function of the 

 ciliated band of the auricularia, which is basically resorbed. A large part of 



