ACTINIAE.— SEA-NETTLES. 185 



which they protect ; at another they contract and 

 disappear before some threatened danger, or they 

 stretch themselves out to seize their almost invisible 

 prey. 



The exterior surface of the actinaria is thickly set 

 with oblong lance-shaped prominences, terminating 

 in a stylet, rigid and sometimes barbed, to wliich is 

 probably due the burning sensation produced by their 

 contact. It is from that sensation that this species of 

 polyps has derived the name of sea-nettles. They 

 have been called anemones, from their resemblance to 

 that beautiful flower, and actinia (starlike) from the 

 rays or tentacles which surround the mouth. 



The coralline polyps are all, like the actinaria, 

 armed with spiculae, of which the forms are most 

 varied, according to the species. Fig. 34 

 shows the general character of these 

 spiculae. The body of the actinia, almost 

 cylindrical when extended, is contrac- 

 tile. It becomes globular, or almost 

 spherical, when the animal is folded 

 back upon itself. Its tentacles are Vn^.'SL 

 then contracted, and almost completely ^Corat' 

 covered by the tough envelope of the 

 body. When extended they serve to arrest by simple 

 contact the little marine animals which touch them. 



The actinia3 do not reside in great depths. They 



