264 zooLoar 



tropical waters and are torn, either by divers or, as in Florida, 

 by hooks of special form, from the rocks at the bottom of the 

 sea near the coast. About 2250 persons are engaged in the 

 sponge fisheries of Florida, and the annual catch is sold for 

 about half a million dollars. The principal centre of the in- 

 dustry is Key West. 



The group of nettle animals is characterized first of all by 

 Uttle organs which, in many species, are capable of irritating 

 even the skin of man ; these organs consist of a little bladder 

 which secretes a poisonous fluid and a long tube by which it 

 passes out of the bladder. The nettle animals also have a true 

 mouth surrounded bj' tentacles. The Hj'dra belong to this 

 group. 



There are two common species of Hj'dra : the one is of a green 

 color { Hydra ^ viiidis -), and the other is flesh-colored (H. 

 fusca''). They are found in standing or slow-running water, 

 attached to submerged plants, sticks, and stones. Throughout 

 the winter they live at the ])ottom of ponds, below the ice. 

 The bod.v of Hydra is soft and highly contractile, so that, when 

 first drawn from the water, it appears hke a speck of jelly. 

 Left undisturbed, the animal ex]:)ands, and its five to eight ten- 

 tacles wave slowly al:)out in search of food. The tentacles are 

 richly supplied with nettling cells, each of which contains a 

 fluid-filled capsule, in which is coiled a thread-like tul^e. 

 When stimulated by contact with some foreign both', the ten- 

 tacle closes around it, while from each capsule the la.sso-thread 

 rolls out as the finger of a glove is rolled inside out, and an irri- 

 tating poison is discharged. If a small animal has been the 

 irritant, it is instantly caught in the thread, jiaral^'zed Ijy the 



^ hydra, a m^'thdlniTical monster, capable of regenerating its head. 

 ^ Green. 3 Brown. 



