163 



oral surface and forms the definitive epithelium 

 (Cameron and Hinegardner, 1974, 1978). After 

 metamorphosis, in most sea urchins the special 

 oral apparatus functions to capture food; it is 

 called Aristotle's lantern. 



During the development of the definitive di- 

 gestive system the larval mouth and anus close 

 before settling, and the tissues of the larval di- 

 gestive system are completely reorganized. The 

 definitive esophagus consists of two parts; inves- 

 tigation of the floor of the amniotic sac (ectoder- 

 mal part) and investigation of the stomach wall 

 (endodermal part). The mouth initially opens in 

 the epineural sac bound by the primary floor of 

 the amnion below and the epineural fold above; 

 the mouth opens outside, in Salmacis bicolor for 

 example, 10-12 days after the juvenile urchin 

 settles (Aiyar, 1936). The intestinal canal, earlier 

 monolayered, now becomes multilayered. The stomach and a part of the 

 intestinal canal become folded. The larval stomach transforms in the anterior 



Figure 116: Structure of 

 pedicellariae in the pluteus of 



Strongylocentrotus nudus. 



bp — base of pedicellaria. 



Figure 117: Diagram of disposition of spicules — the forerunners of interambulacral plates 



in the sand dollars. 



al — anterolateral rod; da — dorsal arch; pd — posterodorsal rod; po — postoral rod; 



spp — spicule-forming plate; sps — spicule-forming spine. 



