ECHIXODERMA 107 



possession of the netted structure. In a plate the princi- 

 pal crystallographic axis is at right angles to the surface, 

 in a spine it is parallel with the length. In fossil specimens 

 the spaces in the network of rods usually become filled 

 with calcite, which is deposited in crystalline continuity 

 with that forming the plate or spine. In such cases the 

 characteristic cleavage of calcite becomes well marked, so 

 that when the plate or spine is broken, the fracture passes 

 along the cleavage planes, instead of being irregular as in 

 the recent forms. By the infiltration of calcite and the 

 development of cleavage, the organic structure in fossil 

 echinoderms is sometimes partly or almost completely 

 destroyed. 



The Echinoderma are divided into two main groups, 

 (1) the Eleutherozoa, (2) the Pelmatozoa. 



I. ELEUTHEROZOA 



The Eleutherozoa possess no fixing organ and are able 

 to move about freely. This group is divided into four 

 classes: — (1) Asteroidea, (2) Ophiuroidea, (3) Echinoidea, 

 (4) Holothuroidea. 



CLASS I. ASTEROIDEA 



The Asteroidea or star-fishes have a flexible body which 

 may be pentagonal in outline, but is usually more or less 

 star-shaped, consisting of a central part, known as the 

 disc, and of five broad arms or rays. In a few forms 

 there are more than five arms, e.g. Solaster papposus has 

 thirteen. 



The mouth is at the centre of the disc on the under 

 surface, and along the under surface of each arm are rows 



