DOWNING] AN AQUARIUM IN A TUMBLER 323 



prey. The larva soon becomes quiescent, the tentacles pull it 

 down towards the mouth at their center, and it is slowly swallowed. 

 It does not go out of sight even then for the body wall of hydra is 

 so thin that it may be seen plainly even after it is entirely within 

 the animal. The mouth of hydra is capable of very wide expan- 

 sion, so that this little beast may take in animals that are appar- 

 ently several times its own size. If the blood worm for instance 

 which it undertakes to swallow is large, one end of the worm may 

 be swallowed and the digestive process will then proceed. After 

 the portion swallowed has been thus digested still more of the worm 

 is crowded down through the mouth into the body cavity. I have 

 seen hydra thus engulf young fish that were three-eighths of an 

 inch long. The animal you see is a hydra-headed monster to these 

 tiny denizens of the ponds where it lives. 



Hydra has been known to scientists for a long time. I have in 

 my library two books devoted to this little animal, both of them 

 written before the middle of the eighteenth century, and even these 

 old zoologists knew a great many of the wonderful things that 

 hydra can do. You can cut the animal in half, either crosswise or 

 lengthwise, and in a few days you will find that each half has 

 remodeled itself into a perfect hydra. This process of regeneration 

 is quite common among the lower animals especially. You can 

 even cut hydra up into fourths or eighths and still the process goes 

 on and you get four or eight complete hydra in place of the one. 

 They are of course small at first but as they feed they soon grow to 

 the size of the adult. If you try this experiment you will hkely 

 fail in a large percentage of your trials because there are bacteria 

 and moulds that grow rapidly on the cut surfaces of hydra and kill 

 some of the specimens that you experiment on, but with due care 

 to see that your dishes are clean and the instruments that you use 

 are clean and that you use clean water, the experiment will succeed 

 in many cases, especially if you will keep the dish covered. The 

 hydra should be well fed also before the experiment begins. 



There are several difi"erent kinds of hydra, one very common one 

 is green, due to the presence in its cells of some tiny plants that live 

 with it as mess mates. The other kinds are brown and some of 

 these brown forms get to be fairly good sized. When expanded the 

 animal may be a half inch long with tentacles that stretch out like 

 tiny hairs three or four inches in length, but even a large one like 

 this when it is disturbed contracts until it is not larger than the 

 head of an ordinary black-headed pin. Not only will a common 



