78 INVEBTEBRATE MOBPHOLOQT. 



body, produces an evagination of the coiled thread from the cyst, 

 in. the interior of which it has been bathed by the fluid contents. 

 The evagination is of sufficient force apparently to puncture the 

 skin of many animals and so inoculate the contents of the 

 cyst, which are of a poisonous nature, and produce inflam- 

 matory disturbances, and in minuter organisms paralysis or 

 death. To the presence of these structures jelly-fishes owe 

 their stinging powers, and they form efficient weapons both 

 for obtaining food and for warding off enemies. 



The Cnidaria may conveniently be divided into three 

 classes : 



I. Class Hydromedusce. 

 II. " ScyphomeduscB. 

 III. " Anthozoa. 



1. Class HYDBOMEDUSiE. 



The HydromedussD present both polyp and medusa forms, 

 the members of some of the orders contained within the 

 class being only of the medusa form, while in another order 

 only the polyp form occurs. Usually, however, both forms 

 occur in more or less perfect development, representing two 

 stages in the life-history of an individual and succeeding one 

 another in the definite manner which has been already 

 described as an alternation of generations. The polyps pos- 

 sess the power of non-sexual reproduction by budding and 

 give rise by this method not only to medusae, but also to 

 other polyp individuals which may remain in connection with 

 one another and thus give rise to branching colonies. As a 

 rule, the medusa individuals separate from the polyp and 

 lead a free life, but in the order Siphonopliorce they may 

 remain in connection with each other and with polyp individ- 

 uals, undergoing various adaptations of form in accordance 

 with different functions which they assume, the whole form- 

 ing a colony presenting in a high degree a division of labor 

 among the component individuals. 



As has already been pointed out the structure of the polyp 

 form differs considerably from that of the medusa. The 

 polyp is more or less cylindrical in form, tapering off above 



