1895 THE MICROSCOPE. 147 



the moath and the anus of the animah To this orifice 

 the arms or tentacles are attached. These tentacles are 

 used to catch the anamalcules on which it preys. They 

 are straight but flexible and easily assume a spiral form, 

 for they are very retractile. They roll themselves around 

 the Daphnies, the lN"ias, and a host of other little ani- 

 mals that live in the water either in a perfect or larval 

 state, for there are without doubt thousands of different 

 kinds of animalcules which inhabit little ponds. When 

 the hydra have swallowed their prey, the body dilates and 

 loses its long form (fig. 3), becoming cylindrical or coni- 

 cal (fig. 4), at the same time the arms contract, making 

 them unrecognisable. 



The hydra hunt like a blind person. They do not 

 throw out one of their arms, like a serpent, against the 

 animal which passes ; but as a spider spins his web so 

 they send in all directions into the water some fine 

 threads or else fix them, in perfect rigidity, to neighbor- 

 ing objects, much like a snare. When an animalcule 

 touches one of these free or fixed threads it is immedi- 

 ately struck with paralysis and remains adhering to the 

 arm which it has touched. The hydra has sufiicient time 

 to surround its captive and draw it into its mouth. 



This phenomenon, so strange and unexpected, is due to 

 the fact that the fresh-water hydra contains some internal 

 organs which secrete a special poison. These consist 

 of a spiral thread connecting with a sac of poison. 

 Every one has heard of the Graleres, that paralyse the 

 bathers, in tropical waters, by their tentacles by giving 

 them urticaria or nettle-rash of sufiicient violence as 

 sometimes to cause death. It is on this account that the 

 sea anemones have received the name of sea nettles from 

 some fishermen. The corals and madreporaria are more 

 or less provided with these poison sacs; some fresh water 

 zoophites also have them. 



If the arms of the hydra are examined with a low 



