157 



Figure 111: Vibratile lobe in the pluteus of Laganum depressum (Mortensen, 1938). 



vl — vibratile lobe. 



ambulacral system of the definitive urchin. Hence, the attachment apparatus 

 and the act of settling are described at the end of the section on metamor- 

 phosis. 



METAMORPHOSIS 



As soon as the larva attains complete development, invagination of the 

 ectoderm takes place from the left side between the postoral and posterodorsal 

 rods. This invagination advances and acquires the shape of a small sac, which 

 separates from the body wall. Its formation was reported earlier by Fewkes 

 (1893), who called it the "Vasoperitoneal sac". McBride (1914) designated 

 this structure the "amniotic sac". It grows and descends toward the stomach 

 up to its contact with the left hydrocoel. A single complex is formed, which 

 comprises the hydrocoel and two-layered wall of the amniotic sac. McBride 

 (1914) noted that in Echinocardium cor datum the amniotic sac does not 

 communicate with the external medium, whereas in larvae of the genus 

 Echinus it forms a narrow canal opening externally. Aiyar (1936), based on 

 McBride, indicated that in larvae of the genus Echinus the coelomic wall is 

 always separated from the stomach by a space, while in Salmacis bicolor, 

 whose development was investigated by Aiyar, the coelomic wall firmly 

 adjoins the stomach. 



Formation of the definitive juvenile occurs in the amniotic sac. This 

 is accompanied by the disintegration of reorganization of larval structures. 



