CLASS ACTIlsroZOA. 



15 



FIG. 20. 



ORDER ACTINOIDA (ak ti noi' da). 



The Actinoida are characterized by having the me- 

 senterial folds and ten- 

 tacles arranged on a 

 plan of six, and by hav- 

 ing the support, when 

 present, of limy matter 

 The common Seas- 

 anemone (a nem'o ne) 

 is often found attached 

 to the spiles of bridges 

 or to the under side of 

 overhanging rocks. 

 When expanded, its 

 gracefully waving, 

 bright colored arms 

 give the animal a 

 plant-like appearance. 

 To the small unwary 

 fish, however, this 

 beautiful growth is 

 most dangerous to ap- 

 proach ; for the slightest contact with one of its ten- 

 tacles will cause the 

 Anemone to throw out 

 a multitude of poisoned 

 barbs. These barbs 

 quickly paralyze the 

 ^ animal which they 

 pierce. 



The tentacles of the 



Me trid'i um mar gin a'tvm. 

 Sea-anemone, as it appears when contracted. O 6 a - a n 6 m One are 



Diagrammatic view of an Ac tin o sd'on, Sea-anemone, 

 one side being removed to show internal anatomy. 

 T, Tentacles. 0, Mouth leading into the oesophagus 

 ?), which in turn opens below into the general body 

 cavity (C). M, A Mesentery. The lower, free ends 

 of others can be seen as radiating lines below the 

 oesophagus. P, Opening in mesentery. W, Body 

 wall, ff, Section of tentacle showing that its cavity 

 is continuous with the body cavity. (Natural size.) 



FIG. 21. 



