162 



Figure 115: Aboral view of the young urchin Strongylocentrotus nudus. 



gp — genital plate; p — pedicellaria; pr — periproct; tp — terminal podium; siam — spines 



of the interambulacral plates. 



No rudiments of the oral apparatus are formed in heart urchins. The oral 

 field is gradually furnished with plates. On the oral side there are five sphaeridia 

 and five buccal podia. 



Feeding : After metamorphosis the nature of urchin feeding changes 

 sharply. Hence the trapping and digestive apparatuses are considerably re- 

 structured. The trapping apparatus of the larvae — the ciliated band — is 

 completely destroyed. Cells of the ciliated band, like the cells of other regions 

 of the larval epidermis, are dedifferentiated, disassociated from one another, 

 and subsequently phagocytized by the cells of the larval digestive system 

 (Chia and Burke, 1978). In the sea urchin Lytechinus pictus the larval arms 

 bend, rudiment of the definitive individual makes contact with the surface, 

 and the former lining of the amniotic sac surrounds the tissues adjoining the 



