846 



THE INHABITANTS OF THE SEA. 



a long thread. A number of barbs or hooks are sometimes 

 disposed spirally around the sheath, the thread itself being often 

 delicately serrated. Under pressure or irritation the thread- 

 cell suddenly breaks, its fluid escapes, and the delicate thread is 

 so rapidly projected that the eye is utterly unable to follow the 



Orticating Organs of Coelenterata. 



*, e.f. Threads and thread-cells of Caryophyllia Smithii. b. Thread-cell of Corynactit AUmani. 



e. Peculiar receptacle of Willsia gteUata, containing thread-cells, d. A single thread-cell of the 



same. g. Thread-cell of Actinia crassicornis.— (All magnffied.) 



process. The violent protrusion of this barbed missile, along 

 with the acrid secretion of the cell, causes many a worm or 

 crustacean of equal or superior strength, that might have gone 

 forth as victor from the struggle of life, to succumb to the ccelen- 

 terate, and is even in many cases exceedingly irritating to the 

 human skin. Besides enabling its possessor to derive his sub- 

 sistence from animals whose activity, as compared with his own, 

 might be supposed to have removed them altogether out of the 

 reach of danger, these stings serve also as admirable weapons of 

 defence, and many a rapacious crab or annelide that would 

 willingly have feasted upon a sea-anemone is no doubt repelled 

 by the venomous properties of its urticating tentacles. 



The Coelenterata have been subdivided into two great classes: 

 the Hydrozoa, in which the wall of the digestive sac is not sepa- 

 rated from that of the cavity of the body, and the Actinozoa, in 



