172 AN INTRODUCTION TO THE STUDY OF FOSSILS 



starfish, and is furnished with rows of well-developed, functional 

 tube-feet. The place of tube-feet on the dorsal side is taken by 

 papillae lacking suckers. Ten large, branched, contractile 

 tentacles, w^hich are probably enlarged tube-feet, surround the 

 mouth. After the manner of some worms, holothurians devour 

 sand and mud, deriving their nourishment from the organic 

 particles contained in them. 



The leathery body is usually supported only by scattered 

 calcareous spicules of various shapes, and possesses no true 

 skeleton. Usually the spicules alone are capable of preserva- 

 tion as fossils ; these are known from the upper Paleozoic to 

 the present, but representatives showing the impression of the 

 entire body have been described likewise from the Middle 

 Cambrian of British Columbia. 



Holothurians are widely distributed at present through 

 all seas, with a habitat extending from shallow to deep water, 

 in tide pools, on rocks, or in the sand or mud. 



Derivation of name. — Greek holothourion, a water-polyp, -f 

 eidos (oid), form. 



1. Name the seven classes of Echinodermata, giving very 

 briefly the distinguishing characteristics of each ; likewise a 

 living and fossil example of each when possible. 



2. What do these classes possess in common that they should 

 be placed in the same phylum ? 



3. How do the Echinodermata differ from the Coelenterata 

 in the following characters : (a) habitat, (b) protecting and 

 supporting skeleton, (c) locomotion, (d) food, (e) its capture, 

 (/) its intake, (g) digestion, (h) blood circulation, (i) excretion of 

 waste, (j) respiration, (k) nervous system, (/) sense-organs, 

 (m) reproduction, (n) geologic range? 



4. Compare the arms of a starfish to corresponding structures 

 in the other classes. 



5. In what class would you look for the ancestor of the 

 stalked forms ? For the ancestor of all Echinodermata ? 



6. In what respects do the Echinodermata show advance 

 over the Coelenterata ? 



