248 COMPARATIVE ZOOLOGY. 
ing canals, and a symmetrical arrangement of all the parts 
of the body around a central axis in multiples of five. 
Fre. 205.—Forms of Echinoderms, from Radiate to Annulose type. 
There are four principal classes, all exclusively marine 
and solitary, and all having the power of secreting more 
or less calcareous matter. 
Crass 1.—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 internal skeleton for strength 
and support, the entire animal, body, arms, and stem, con- 
sisting of thousands of stellate pieces connected together 
by irritable matter. Crinoids were very abundant in the 
old geologic seas, and many limestone strata were created 
out of their remains. They are now nearly extinct: dredg- 
ing in the deep parts of the Caribbean Sea and Atlantic 
Ocean has brought to light two or three living representa- 
tives. 
