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 
