Echinodermata. 47 



ment of parts, the bodies being formed of a series 

 of symmetrical segments around a central axis. In 

 all of them, the body wall of each individual animal is 

 made up of two membranes, an outer and an inner ; all 

 are aquatic, and, with about four exceptions, marine. 

 They all have a central stomach, a mouth at one pole, 

 surrounded by tentacles which are armed with thread- 

 cells, these latter being almost universal in the sub- 

 kingdom. When a nervous system exists it is as an 

 obscure ring, and is related to the margin at the base 

 of the tentacles. 



The two great classes of Coelenterates, Hydrozoa 

 and Actinozoa may be contrasted thus : the former 

 have.but one internal cavity ; the latter have a central 

 stomach cavity surrounded by a separate, though 

 communicating body cavity, and the egg-producing 

 organs open into this second space. 



The two great sub-classes of Actinozoa are 



A, Those with simple tentacles in multiples of 5 

 or 6 : Sub-class i, Zoantharia. 



B. Those with pinnate tentacles in multiples of 4: 

 Sub-class 2, Alcyonaria. 



CHAFFER IX. 



SUB-KINGDOM 4. ECHINODERMATA I ENCRINITES 

 AND STAR-FISHES. 



General Characters. Sea-urchins, star-fishes, and sea- 

 cucumbers make up a natural assemblage of animals, 

 called Echinodermata on account of the spiny skins 



