CCELENTERATA. 



163 



discharged 



the ce!i ad 



which is drawn out into a long and slender tube coiled 

 up inside of the rest. These nettle-cells can be 'dis- 

 charged' by the animal, the discharge consisting in a 

 forcing out of the tube in the same way in which one 

 may blow out the inturned finger of a 

 glove. These cells contain a strongly 

 irritant poison, and at the discharge 

 this poison escapes. These nettle- 

 cells furnish a means of defense, and 

 they are also used in obtaining food, 

 the poison being strong enough to 

 paralyze small animals instantly. In 

 some forms it is -strong enough to 

 affect man. For instance, the tenta- FlG 

 cles of the Portuguese man-of-war will 

 quickly raise a bright-red ridge on 

 the hand or arm of man and produce an almost intolerable 

 burning sensation in the parts thus touched. 



In many Coelenterates there is no specialized nervous 

 system, the general surface of the body having sensory 

 and nervous powers. In others there is a central nervous 

 system arranged in a ring around the body; and some 

 of the jelly fishes have organs the structure of which 

 leads to their being regarded as simple types of eyes and 

 ears. 



Some move about freely, some are as firmly fixed as is 

 any plant; but both fixed and free conditions may occur 

 in the life-history of a single species. Some of the fixed 

 forms may be single, but others will form buds upon 

 the sides or from the point of attachment; and then 

 these buds will grow tentacles, while a mouth will open 

 at the tip of each, so that there results a compound animal 

 with many parts which are more or less complete repeti- 



