C(ELENTERATA. 311 



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 instantly small animals. 

 In some forms it is strong enough to affect man. For in- 

 stance, the tentacles 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 may have organs, the structure of which leads to 

 their being regarded as simple types of eyes and ears. 



With very few exceptions, the Ccelenterates are marine. 

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

 plant ; but, as has already been explained (p. 305), both fixed 

 and free conditions may occur in the life-history of a single 

 species. All of the Coelenterates reproduce by means of 

 eggs, but, besides, most forms have the power of forming 

 buds which grow into new individuals, sometimes like, 

 sometimes greatly different from, tbe parent. The Coelen- 

 terates are divided into two classes, accordingly as they 

 possess or lack an inturned oesophagus. 



CLASS I. HYDROZOA (p. 305). 

 CLASS II. SCYPHOZOA (p. 301). 



