64 BRANCH ECHINODERMATA 



intestines. The interradial anal opening (see Fig. 49) is 

 situated near the mouth. 



Multiplication. — Crinoids multiply by eggs, which pass 

 through complex changes before reaching the adult stage. 



Habitat. — The living crinoids are deep sea animals with 

 the exception of two genera, which live at a less depth. Some 

 have been dredged from a depth of 11,100 feet. At this depth 

 the water pressure must be enormous. 



Geologic Distribution. — Primitive types (the cystids and 

 blastoids) of this group are among the most ancient fossils. 

 True crinoids appeared before the close of the Cambrian 

 Period. They reached their culmination in the Carboniferous 

 Period. The crinoid fossils of this period are so numerous that 

 many beds of limestone are composed principally of them. 

 Burlington, Iowa, and Crawfordsville, Indiana, are noted for 

 their numerous and well-preserved fossil crinoids. Crinoids, 

 though formerly of such vast numbers, are now almost extinct. 



Important Biologic Facts. — Echinoderms arc radially sym- 

 metric, but embryology shows that they have developed from 

 the bilateral tyj^e. It is reasonable to regard those classes of 

 echinoderms as the more ancient which have the radial sjTn- 

 metry less completely developed.' 



The locomotor-ambulacral system is found in no other 

 branch. 



The echinoderms are a singularly isolated group, and we 

 look in vain among the known members, living and fossils, of 

 other branches for any really close allies. 



Classification. — 



Class. Examples. 



I. Asteroi'dea. Starfishes. 



II. Ophiuroi'doa. " Brittle-stars." 



III. Echinoi'dea. Sea-urchins. 



IV. Holothuroi'dea. Sivi-cucumbers. 



v. Crinoi'dea. Sea Lilies, " Feather-stars." 



VI. Cystoi'dea. Fossil. 



VII. Blastoi'doa. Paleozoic fossil, as in Class VI. 



> Parker and Haswcll's " Zoology,'' vol. i, p. 401. 



