PHYLUM VI. ECHINODERMA. 



The term Echinoderma means spiny skin, and both star- 

 fishes and sea-urchins possess this peculiarity in a high 

 degree. But besides this external characteristic there are 

 manv other features which dis- 



/ 



tinguish the group. In fact, 



there is scarcely a division in 



the whole animal kingdom more 



sharply marked off from other 



forms than this. In all the 



body is built on that radiate 



plan which is so prominent in 



starfish and urchin, and in all 



except a few starfish there are 



five rays, although in some 



the rays may subdivide. This 



radiate condition affects not 



only the external surface, but 



may extend to every system as FIG 90 _ Larva of a starfishi 



well. And yet we may trace in larged. m, mouth; i>, vent. 



every form a bilaterality, and development shows that the 

 bilateral condition is primitive, for the larvae (see fig. 90) 

 clearly have the two sides alike, while the radial symmetry 

 of the adult only appears later in the growth. It was this 

 radial arrangement of parts which formerly led to the union 



of Echinoderma and Ccelenterata as a branch Radiata. a 



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