168 



Figure 122: Development of the definitive digestive system in Echinus esculentus (McBride, 



1903). 



A — larva III stage; B — : juvenile; a — anus, des — definitive esophagus; hg — hind gut; 



les — larval esophagus; st — stomach. 



Figure 123: Formation of processes 

 in the amniotic sac in 

 Echinocardium cordatum 



(McBride, 1914). 



as — amniotic sac; cp — coelomic 



processes. 



rac 



Figure 124: Formation of ambulacral 

 ring canals and radial canals upon 

 branching of hydrocoel in 

 Echinarachnius parma (Fewkes, 



1893). 



arc — ambulacral ring canal; cp — 



coelomic processes; pd — posterodosal 



rod; rac — ambulacral radial canal. 



requires the presence of shell fragments previously attached to the adult 

 animals. Mortensen (1921) observed a delay in settling of the larvae of 

 Mellita sexiesperforata in the absence of a sandy substrate. A peptide respon- 

 sible for the metamorphosis of larvae has been isolated from the tissue ex- 



