BRANCH ECHINODER'MATA 



Plan of Structure. — These animals are characterized by their 

 five-rayed or pentameral plan of structure. AA'hile the echino- 

 derm is radially symmetric, the development shows that it is 

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



■■"^ 



I^ig. SQ— Solas' ler 



cndcra (small sppoimen, natural size), oral view. 

 (Bulletin, U. S. F. C, 19(12.) 



central portion is the df'.s/i, from which arms or rays project, as 

 seen in the starfish. Close t'xamination will r('\'('al this penta- 

 merous plan in the sea-urchin antl in the s(-a-cucumber. For, 

 suppose the rays of the starfish were flexed and their ediies 

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



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