124 



mda 



Figure 83: Initiation of radial canals of the ambulacral system in the brachiolaria of Asterias 



vulgaris attached to the substrate by brachiolar arms (Goto, 1896*). 

 abr — anterior brachiolar arm; ada — anterodorsal arm (projection); es — esophagus; 

 he — hydrocoel; Ibr — lateral brachiolar arm; mda — mediodorsal arm (projection); 

 pda — posterodorsal arm (projection); pla — posterolateral arm (projection); poa — postoral 

 arm (projection); proa — preoral arm (projection); st — stomach; 1 to 5 — rudiments of 

 radial canals of ambulacral system. 



mesodermal origin. The first skeletal plates — central, five radial, terminal, 

 and five interradial basal — form in the region of separation of the aboral 

 disk of the definitive asteroid. One basal plate becomes the madreporite, 

 which closes the outer opening of the stone canal of the ambulacral system. 

 Other elements of the skeleton appear later (Figure 84). 



Rudiments of definitive organs : In the juvenile sea star, all systems of 

 the definitive body are found in various stages of formation; these become 

 fully developed as the asteroid grows. Within a few days after metamorpho- 

 sis, all these systems become functional except the reproductive. Develop- 

 ment of the reproductive system has not been hilly investigated. Some authors 



