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tentacles and five radial nerves extending along the radial ambulacral canals. 

 The ring canal develops from the thickening of the vestibular bottom (Kume 

 and Dan, 1968). According to Ivanova-Kazas (1978), the presence of a con- 

 nection between the larval and definitive nervous system is assumed. This 

 assumption is based on the data of Metchnikoff, who observed that before 

 metamorphosis the nerve elements present in the auricularia in the preoral 

 field shift to the oral opening and become a part of the vestibular bottom of 

 the doliolaria. Semon (1888) was also of this view. 



Statocysts lie at the base of the radial nerves of the pentactula; possibly 

 they are involved in larval settling. 



Settling : After the appearance of spicules, the pentactula loses its ciliated 

 bands and finally settles down. Now the formation of secondary tentacles and 

 ambulacral podia commences. In the literature there is no information about 

 the mechanism of settling of sea cucumber larvae. Nothing specific is known 

 about their ability to recognize a suitable substrate for settling. Chia and 

 Spaulding (1972) mentioned that the polychaete tubes of Phyllochaetopterus 

 induce settling in Cucumaris miniata and Psolus chitinoides . Slime stimulates 

 metamorphosis in Molpadia intermedia (McEuen and Chia, 1985). 



The pentactulae of all sea cucumbers have primary tentacles. In addition 

 to them, the species with ambulacral podia, develop one or two podia by the 

 time of settling. Strathmann (1978a) reported that various synaptids which 

 lack ambulacral podia in the adult stage, use their primary tentacles for 

 digging into the ground or creeping. 



In the pentactula stage, Stichopus japonicus is capable of attaching to the 

 substrate by means of a single ambulacral podium which, by this time, has 

 appeared at the posterior body end. The larva, on attachment with this po- 

 dium, remains sessile for some time, then retracts the podium within its body 

 and resumes swimming. It repeats such maneuvers until the primary tentacles 

 are finally formed and the vestibule is reduced. After this, the podium is no 

 longer retractable, but the larva continues for some time to alternate swim- 

 ming over the substrate with creeping on it. While swimming, it touches the 

 substrate from time to time. 



Based on the information available on the structure of the terminal podia 

 of sea urchins (Burke, 1980) (see Figure 127), it may be assumed that the 

 ambulacral podia in sea cucumbers have an analogous structure. If the am- 

 bulacral podia of sea cucumbers have sensory cells, they ought to play an 

 important role in the selection of the substrate and settling. Judged from the 

 observations of various researchers, the primary tentacles are capable of 

 discharging the same role. 



