Chap. 32 ECHINODERMS FORERUNNERS OF THE VERTEBRATES 653 



falling upon them. Suffocation from these is prevented by a remarkable skin- 

 cleaning and trapping equipment consisting of great numbers of minute pincers 

 distributed over the outer surface of the body (Fig. 32.3). The smallest adult 

 echinoderm is half an inch in diameter. The largest starfish is 32 inches or more 

 across and a slender worm-shaped sea-cucumber may be six feet long. 



Class Crinoidea — Sea Lilies 



In ancient times crinoids {crinon meaning lily) stood like waving lilies 

 attached to primeval sea bottoms (Fig. 29.4). Some of those sea bottoms were 

 long ago lifted and now constitute inland highlands. In upper New York 



starfish 



sea urcnin 



ECHINOIDEA 



OPHIUROIDEA 



CRINfqiDEA sea 



cucumber^^ 



HOLOTHURIOIDEA 



Fig. 32.2. Schematic representation of the relations of important structures in 

 five classes of echinoderms. T, tube feet; 5, spines; M, mouth; A, anus. (Courtesy, 

 Storer: General Zoology, ed. 2. New York, McGraw-Hill Book Co., 1951.) 



State, slabs of stone have been found with dozens of fossil sea lilies pressed 

 into it as if they had been outspread by falling earth. The majority of fossil 

 echinoderms are crinoids but there are relatively few living species. These are 

 the stalked sea lilies attached on the bottom in deep waters and the feather 

 stars that can swim feebly on the surface. All crinoids are attached when young 

 (Fig. 32.1). Each arm and its branches bear central grooves through which 

 cilia propel particles of food toward the upturned mouth. 



Class Asteroidea — Sea Stars or Starfishes 



These echinoderms have flat bodies with five conspicuous arms or varia- 

 tions of this number. The central part of the body is relatively broad and high 

 and the arms short. In all starfishes, the arms are broad as compared with 

 those of brittle stars. 



Class Ophiuroidea — Brittle or Serpent Stars, Basket Stars 



The body is flattened with a very definite central disk from the under side 

 of which the slender, flexible, jointed arms are sharply marked off. These are 

 provided with strong muscles and are bent and lashed like rapidly moving 

 serpents. There are no tube feet, but structures comparable to them all help 

 to pass food toward the mouth. Brittle stars are the most agile of echinoderms, 

 crowding together in narrow crevices and scuttling rapidly when disturbed. 



