88 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



parts of the txody around a central axis in multiples of 

 five, 14 this radial arrangement, however, concealing a 

 definite bilateral symmetry. They are, thus, much 

 more highly organized than the coelenterates, with 

 which group they have very little in common except 



FIG. 45. Forms of Echinoderms, from radiate to annulose type: a, Crinoids: b, Ophi- 

 urans; c, Starfish; d, Echini; e, Holothurians. 



their apparent radial symmetry. In the course of 

 development in echinoderms metamorphosis occurs, the 

 larval forms bearing no resemblance to the adults. 



There are five principal classes, all exclusively marine 

 and solitary, and all having the power of secreting more 

 or less calcareous matter to form the skeleton. 



CLASS i. Asteroidea 



Ordinary starfishes consist of a flat central disk, with 

 five or more arms, or lobes, radiating from it, and con- 

 taining branches of the viscera. The skeleton is leathery, 

 hardened by small calcareous plates (twelve thousand by 

 calculation), but somewhat flexible. The mouth is below ; 

 and the rays are furrowed underneath, and pierced with 



