DISTRIBUTION OF ORGANISMS. 



169 



the higher genera of the latter family, the tentacles are short- 

 ened and reduced in number. In the Ophiurce, there are only 

 five long and simple rays projecting from the central body. 

 Afterwards, in the Aster iadce, or true star-fishes, the central 

 part dilates step by step, until it fills up the interstices between 

 the rays, and the form becomes a pentagonal disk. From this 

 there is a clear passage to the Echinus or sea urchin, which is 



FIG. 83. 



Echinus mammillatus (recent). 



merely a spheroidal animal in a calcareous case, from which 

 numberless spines and tentacles project, for locomotion and the 

 collection of food. This form again becomes elongated into 

 the cylindrical soft-bodied Holothuria, with a circle of tenta- 

 cles at the oral extremity ; thence the transition is easy to the 



FIG. 84. 



ffolothuria phantapus. 



