ECHINODERMATA. 65 



contours and in possessing the netted structure. In a 

 plate the principal crystallographic axis is at right angles 

 to the surface, in a spine it is parallel to the length. In 

 fossil forms the spaces in the network of rods generally 

 become filled with calcite, this being 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 the fossils is sometimes more or less com- 

 pletely destroyed. 



Reproduction in the echinoderms is mainly sexual, 

 and as a rule the sexes are separate, but they do not 

 differ externally. The Echinodermata are divided into the 

 following seven classes: (1) Asteroidea; (2) Ophiuroidea ; 

 (3) Echinoidea; (4) Holothuroidea ; (5) Crinoidea; (6) 

 Cystidea; (7) Blastoidea. 



CLASS. ASTEROIDEA. 



The star-fishes constitute the class Asteroidea. In 

 these the body is sometimes pentagonal in outline as in 

 Goniaster, but in the majority of cases it is more or less 

 star-shaped, consisting of a central part, the disc, which is 

 produced into five arms or rays. In a few forms there are 

 more than five arms, as in Solaster papposus which has 

 thirteen. 



On the under surface of the animal are placed the 



mouth and tube-feet. The mouth is in the centre of the 



disc, and along the arms are rows of tube-feet, these are 



tubular processes connected with the radial water-vascular 



w. P. 5 



