48 LEISURE-TIME STUDIES. 



fashion, or, like the " looping " caterpillars, by alternately 

 fixing and extending the mouth and root-extremity of the 

 body whilst occasionally they may be seen to float listlessly, 

 with extended tentacles, amid their native waters. 



When any minute animal, such as a water-flea, or some 

 similar organism, comes in contact with the tentacles of the 

 hydra, an interesting series of acts is witnessed. The, ten- 

 tacles are then observed to act as organs for the capture of 

 prey ; the victim being seized and conveyed by their contrac- 

 tion towards the mouth of the animal, within which cavity 

 it finally disappears from view. That the hydra, therefore, 

 possesses instincts common to all forms of animal life, high 

 and low alike, and which lead it to supply the wants of its 

 frame, cannot be doubted ; and Schiller's maxim, that 

 hunger is one of the powers that rule the universe, may thus 

 be aptly illustrated within the small domain and in the 

 simple life-history of the hydra. 



As might be expected, the prey at first struggles violently 

 to escape from the clutches of its captor ; but after a short 

 period the struggles become less marked, and the captured 

 animal may be noted to become somewhat suddenly help- 

 less and paralysed. The observation of these details leads 

 us to expect that the hydra possesses some offensive appa- 

 ratus, through the action of which the capture of prey is 

 facilitated. And an examination, by aid of the microscope, 

 of the tentacles of the polype, and in fact of its body-sub- 

 stance as well, would reveal the presence of numerous 

 minute capsules, named " thread-cells," which are developed 

 in the general tissues of the body. Each of these curious 

 little cells consists of a tough outer membrane, within which 

 a delicate thread or filament lies coiled up amidst fluid. 

 When one of these structures is irritated, as by pressure, the 

 cell is observed to rupture, the thread being thrown out or 

 everted, and the fluid at the same time escapes. A thread-cell 

 of the hydra in its ruptured condition appears as an oval 

 capsule, having attached to one extremity the thread, which 

 is provided at its base with three little spines or hooks. The 



