ioo ELEMENTARY ZOOLOGY 



in times of danger, to the other structures. These pro- 

 tecting-scales are greatly modified medusa-zooids, each 

 consisting of a simple cartilage-like gelatinous mass 

 penetrated by a food-carrying canal. Under the broad 

 leaves of these protecting-zooids are a number of pear- 

 shaped bodies which have a wide octagonal mouth-open- 

 ing at their free end, and possess in their interior certain 

 digestive glands. Each one is provided with a very long 

 flexible tentacle which bears many fine stinging-threads. 

 The tentacle waves back and forth in the water, and 

 on coming in contact with an enemy or with prey its 

 poisonous stinging-threads shoot out and paralyze or 

 wound the unfortunate animal. These pear-shaped bodies 

 are the feeding structures, each being a modified polyp- 

 zooid. Scattered among these dangerous structures are 

 many somewhat similarly shaped but wholly harmless 

 structures, the sense-structures. Each of these has a 

 pear-shaped body but without mouth-opening, and also 

 a long, very sensitive, tentacle-like process. The sense 

 of feeling is highly developed in these tentacles, and they 

 discover for the colony the presence of any strange body. 

 These sense-structures are modified polyp-zooids. Finally 

 there are two other kinds of structures, usually arranged 

 in groups like bunches of grapes, which are the repro- 

 ductive structures, male and female. They are modified 

 medusa-zooids grown together and without tentacles. 

 This whole colony, or this compound animal, floats or 

 swims about at the surface of the ocean, and performs all 

 of the necessary functions of life as a single animal com- 

 posed of organs might. Yet the Siphonophore is more 

 truly to be regarded as a community in which the hundreds 

 or thousands of animals, representing five or six kinds of 

 individuals, all of one species, are fastened together. Each 

 individual performs the particular duties devolving upon 

 its kind or class. Thus there are food-gathering indi- 



