ECHINODERMATA. 73 



Phylum II. ECHINODERMATA. 



Radial marine animals with pentamerous symmetry (the larvae bilater- 

 ally symmetrical), and with calcifications in the skin. A water- vascular 

 system typically composed of a ring-canal with connected stone-canal, 

 Polian vesicles, tentacular tubes, and five ambulacral canals. A nerve- 

 ring with five main trunks, radial in position. After complete (often 

 regular) cleavage of the fertilized ovum a blastula with segmentation- 

 cavity is formed, and from this a gastrula with the two germinal layers, 

 ectoderm (epiblast) and endoderm (hypoblast). The mesoderm and 

 water- vascular system are formed from diverticula of the archenteron. 

 The blood-vascular system is developed from spaces in the mesodermic 

 tissue. Body-cavity an enteroccele. A metamorphosis. 



