THE PHYLA HEMICHORDATA AND ECHINODERMATA 



371 



ment in sea lilies is limited to postural changes of the body and the 

 spreading or folding together of the branches. Although 5000 extinct 

 species have been described, only 80 living species of attached crinoids 

 are known. 



The sea lilies were first known as fossils. In the 19th century, shortly 

 after evolution became an accepted theory, a number of scientists sug- 

 gested that living representatives of extinct groups might still be found 

 in the ocean depths. W^hen the first dredging explorations into these 

 depths yielded living sea lilies, there was much excitement and hope 

 that other "living fossils" would be found. The failure to find animals 

 such as trilobites was a disappointment; although a number of survivors 

 of groups that are mostly extinct have been found in deep water, the 

 number is not much greater than that found in other regions. 



Another 550 living species of crinoids occur in a recently evolved 

 family that are free-living as adults. These are the feather stars (Fig. 

 19.10) in the family Comatulidae. They attach as larvae and grow a 

 short stalk like that of the sea lilies, but later break loose. Their general 

 anatomy and method of feeding are unchanged. Feather stars differ from 

 the sea lilies primarily in locomotion. They can crawl through the 



Figure 19.10. The feather star, a crinoid that lacks a stalk as an adult. (Austin H. 

 Clark: in Smithsonian Misc. Coll., Vol. 72, No. 7.) 



