l68 AQUATIC MICROSCOPY FOR BEGINNERS. 



ing the creatures in an aquarium. They never eat any 

 but animal food, and they are always hungry. 



The body and tentacles of Hydra viridis are rough- 

 ened by little elevations or warty prominences. The 

 brown species {H. vulgaris) is not so much roughened. 

 These warts contain what are called the stings. These 

 are small oval or vase-shaped hollow bodies, with a 

 fine thread coiled in the interior, and four minute spines 

 near the summit. When the Hydra is irritated by the 

 pressure of the cover-glass these stings are thrown out 

 violently, and the long stiff thread can be 

 well seen. When in the animal's body 

 they cannot be easily examined. One is 

 shown much magnified in Fig. 131. They 

 are often found on the slide when no Hydra 

 iA is to be seen, and they are sometimes hotice- 

 \j able sticking in the body of some worm or 

 Fig. 131.— larva that has escaped a fatal embrace. I 



Hydra sting , 



have more than once found a Chironomus 

 larva (Chapter VII.) in a dying condition and ornamen- 

 ted by a spiral band of these stings in its skin, it hav- 

 ing evidently had a tussle with a Hydra and escaped. 

 The Hydra increases in numbers rapidly by a process 

 of budding. A little protuberance appears on one side 

 of the body, enlarging and growing, and finally, while 

 still attached to the parent, developing tentacles, then 

 resembling the mature animal in everything except in 

 size. And it is not unusual to see one or more still 

 younger Hydras sprouting from these before they are 

 free from the parent. The body of the young Hydra 

 is hollow, and communicates with the body-cavity of 

 the parent. It captures food like the parent, and it is said 

 to be no uncommon sight to see the old and the young 



