258 COMPARATIVE ZOOLOGY. 



system of circular and radiating canals, connected with the 

 outside water by means of the madreporic tubercle, and a 

 symmetrical arrangement of all the parts of the body around 

 a central axis in multiples of five. 133 There are four princi- 

 pal classes, all exclusively marine and solitary, and all hav- 

 ing the power of secreting more or less calcareous matter. 



CLASS I. Crinoidea. 



The Crinoids, or " Sea-lilies," are fixed to the sea-bottom 

 by means of a hollow, jointed, flexible stem. On the top 

 of the stem is the body proper, resembling a bud or ex- 

 panded flower, containing the digestive apparatus, with 

 the surrounding arms, or tentacles. The mouth looks up- 

 ward. There is a complete skeleton for strength and sup- 

 port, the entire animal body, arms, and stem consisting 

 of thousands of stellate pieces connected together by liv- 

 ing matter. Crinoids were very abundant in the old geo- 

 ologic seas, and many limestone strata were formed out of 

 their remains. They are now nearly extinct: dredging 

 in the deep parts of the oceans has brought to light a few 

 living representatives. 



CLASS II. Asteroidea. 



Ordinary Star-fishes 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 

 numerous holes, through which pass the sucker-like tenta- 

 cles the organs of locomotion and prehension. The red 

 spots at the ends of the rays are eyes. The usual color of 

 Star-fishes is yellow, orange, or red. They abound on ev- 

 ery shore, and are often seen at low tide half buried in 

 the sand, or slowly gliding over the rocks. Cold fresh 



