BRANCH ECHINODER'MATA 



Plan of Structure. — These animals are characterized by their 

 five-rayed or pentameral plan of structure. While the echino- 

 derm is radially symmetric, the development shows that it is 

 derived from the bilateral type. The larvce are bilateral. The 



Fig. 39. — Solas' ter endeca (small specimen, natural size), oral view. 

 (Bulletin, U. S. F. C, 1902.) 



central portion is the disk, from which arms or rays project, as 

 seen in the starfish. Close examination will reveal this penta- 

 merous plan in the sea-urchin and in the sea-cucumber. For, 

 suppose the rays of the starfish were flexed and their edges 

 joined, the form of the sea-urchin would appear. Again, 



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