138 ACTINOZOA. 



Contact of food^ mechanical irritation, or the 

 withdrawal of the sea-water within which it dwells, 

 causes the Actinia to assume its contracted con- 

 dition. In this state it appears as a somewhat 

 conical, inert mass, often much flattened, the 

 mouth and tentacles being more or less completely 

 concealed by the folded margin of the disc. In the 

 act of expanding, this is gradually rolled backwards, 

 displaying the tentacles, which, as the margin con- 

 tinues to unfold itself, are soon distended to their 

 full extent by the pressure of the fluid contained 

 in the somatic cavity. 



The Actinia has the power of effecting con- 

 siderable alterations in the general form of its 

 body by the alternate contraction and expansion 

 of the muscular fibres mentioned above. It can 

 also, like the Hydra, shift its position at pleasure, 

 though some species, under ordinary circumstances, 

 attach themselves so firmly as not to be removed 

 without laceration of the base. 



2. Oeneral Morphology. — In no essential 

 respect does any Actinozoon depart from the typi- 

 cal structure above described, nor do the members 

 of the present group present such varied modifi- 

 cations of a common plan as have been shown to 

 appear in the Hydrozoa. In all Actinozoa the 

 digestive apparatus, though communicating freely 

 with the somatic cavity, is furnishe'd with a wall 

 of its own, between which and the outer boundary 

 of the body the generative elements are produced. 

 By these characters they may readily be distin- 

 guished from the Hydrozoa, with which, in the 

 more minute details of their structure, they closely 

 agree; the body of an Actinozoon, like that of 



