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 ectoderm and 

 an inner endoderm ; all are aquatic, and, with about 

 six 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 ner- 

 vous 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 Ccelenterates, 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. 



CHArTER IX. 



SUB-KINGDOM IV. ECHINODERMATA : 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 



