142 OTHER ECHIXODERMS 



Exercise 2. — Other Classes. 



(b) Examine specimens of brittle-stars, Ophiuroidea; of sea- 

 urchins, Echinoidea; of sea-cucumbers, Holothuroidea; of sea-lilies, 

 Crinoidea; and specimens of fossil crinoids. Compare them as to the 

 five-parted, radial symmetry; position of the body with reference to 

 environment; and other features. In no other phylum is there such 

 diversity in the relationship of body axis to the environment. Com- 

 pare diagrams of the internal structure of starfish, sea-urchin, and sea- 

 cucumber. 



