346 - THE INHABITANTS OF TUE SEA. 
a lung 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 
Urticating Organs of Coelenterata. 
2,e. f. Threads and thread-cells of Caryophyllia Smithii. 6. Thread-cell of Corynact?s Allmani. 
c. Peculiar receptacle of Wilisza stel/ata, containing thread-cells. d. A single thread-cell of the 
same. g. Thread-cell of Actinia crassicornis.—( All magnified.) 
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 ceelen- 
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 Ceelenterata 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 
