28 Transactions of the American Institute. 



ccll<ti are bell-shaped animals, resembling flowers, eacli fixed by a 

 delicate and flexible stem ; the zootliamnia are flowering trees, in 

 Avhich a multitude of ramifications proceed from a single trunk equally- 

 flexible; and though all these exhibit excessive irritability, contract- 

 ing themselves suddenly on the slightest alarm, yet in this fact there 

 is nothing to which we have not something like a familiar parallel in 

 the mhnosa setisitiva, or the flower called Ve?iifs^s fly-trap among 

 well known plants. The zoothainnia also grow like plants putting 

 out new branches from time to time, till froni a single individual 

 there spring up scores, all having a separate, and, at the same time, a 

 common life. 



This plant-like mode of increase characterizes indeed very strikingly 

 an order of animals which, though microscopic, are higher than the 

 mere infusoria. The hydrozoa, or fresh-water polyps, not only pre- 

 sent a plant-like appearance, but put forth buds like plants, which 

 buds become developed into perfect polyps themselves, and then 

 separate from the parent to seek an independent life. These polyps 

 are little animated sacks, fixed by the base, and having their own 

 mouths surrounded by a number of arms or tentacles, which are the 

 organs by which they seize their prey. The power of contractility 

 and extension of these arms is surprising. In length they exceed 

 often enormously the body of the animal, which, however, possesses 

 the power of withdrawing them until they almost disappear. Often 

 from the same polyp several germs will be seen to be simultaneously 

 springing, and these will sometimes put forth secondary buds of their 

 own before they become separated from the parent stem. 



Another remarkable characteristic of this polyp is the tenacity of 

 its vitality, and its power of repairing au}'^ injury which it may suffer 

 by violence. If it be deprived of its tentacles they will speedily be 

 reproduced. If the whole head be cut off' a new head will presently 

 be formed ; and, more than that, the severed head will presently 

 provide itsel ' with a new body. Some curious experiments of this 

 kind were made by Mr. Trembley, the first observer of this interesting 

 object. He cut. an individual longitudinally into two equal parts, 

 leaving the parts slightly attached at the lower extremity. Each 

 half speedily replaced the half it had lost, and thus were formed two 

 complete animals, remaining still united at the base. The experi- 

 ment has been earned still further, A hydra has been cut into nearly 

 forty fragments, and every one of the fragments became, in a short 

 time, an entii'e animal. 



