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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



venom. The prey of these polyps has little 

 chance of escape from the attack of these cells ; 

 since wounded by the threads, which doubtless 

 become attached to its body by the hooks, and 

 poisoned by the fluid, even animals of tolerably 

 large size, when compared with the hydra, may 

 be seen to succumb to its attack. The polyps 

 are thus seen to be singularly well provided as 

 regards offensive apparatus, the particular form 

 and action of which remind one, in some degree, 

 of the famous " lasso " of Western nations. And 

 ' it is at the same time interesting to note that 

 thread-cells of essentially similar nature to those 

 found in the hydra confer on the jelly-fishes and 

 allied forms the stinging powers which render 

 these beings the terror of tender-skinned bathers. 

 The internal structure of our polyp is of the 

 simplest possible description. It may seem 

 strange to talk of an animal body which lives 

 and grows without any of the structures or ma- 

 chinery we are accustomed to associate with the 

 higher animals. Yet the hydra exemplifies the 

 former condition ; since we might accurately 

 enough describe its body as consisting of a sim- 

 ple tube, the interior of which contains no organs 

 of any kind, and which communicates with the 

 outer world through the mouth. If we further 

 suppose that the walls of this tubular body are 

 composed of two closely-applied layers or mem- 

 branes — the outer somewhat dense and tough, 

 and the inner of more delicate nature — we shall 

 have formed a broad but accurate idea of the 

 constitution of these polyps. When the prey or 

 food is swallowed, it therefore passes into the in- 

 terior of the tubular body, which evidently serves 

 as a stomach-sac. Here the morsel is digested 

 or dissolved, and, as the result of this process, 

 a fluid perfectly adapted for the nourishment 

 of the polyp is formed. This fluid or blood is 

 kept circulating throughout the interior of the 

 simple body, by the constant movements or 

 vibrations of numerous minute processes named 

 cilia, which exist like a fringe on the lining mem- 

 brane of the body-cavity, and which therefore 

 perform the functions of the heart of higher ani- 

 mals. Thus it may be said that every part of the 

 hydra's body is brought directly into contact 

 with this nutritive fluid, since we note that the 

 fluid is transmitted from membrane to membrane 

 and from cell to cell by the process of imbibi- 

 tion. And in this simple manner does the hydra 

 repair the continual waste of its parts ; this pro- 

 cess of waste being the inevitable result of the 

 acts and functions of every living being, and the 

 invariable concomitant of life itself. 



We have already noted that the hydra pos- 

 sesses the power of appreciating sensations, since 

 it shrinks when touched, and exhibits other 

 proofs of its sensitiveness. In the possession of 

 this power the polyp resembles some plants, 

 and most, if not all other animals, including man 

 himself. Broadly stated, this power which the 

 hydra possesses may be regarded as presenting 

 us with the idea of a nervous system in its sim- 

 plest phase. The functions of such a system may 

 be summarized in the statement that it is adapt- 

 ed for bringing the animal into relation with its 

 surroundings. We thus say that the nervous 

 system exercises the function of "relation;" 

 while, from the fact that the animal performs this 

 function through impressions being made upon 

 it, we are also accustomed to speak of the ner- 

 vous power as exercising the function of " irri- 

 tability." This power, in fact, stands mediately 

 between the animal and the world in which it 

 lives. The higher we ascend in the animal 

 scale, the more perfectly do we find the nervous 

 system adapted for placing the animal in posses- 

 sion of a knowledge of its environments; al- 

 though, as will be presently remarked, the dif- 

 ferences between the nervous powers of higher 

 and lower animals are to be considered rather 

 differences of degree than of kind. 



But, as an examination of the hydra demon- 

 strates to us, the view just taken of the nervous 

 functions can hardly be considered of a complete 

 kind. For we find that the polyp when touched 

 is enabled to act upon the knowledge or sensa- 

 tion which the touch conveys ; since its tentacles 

 contract, and its whole body shrinks as if in irri- 

 tation and alarm. The reception of a sensation 

 by the nervous system is therefore accompanied 

 by a power of acting upon " information re- 

 ceived ; " and it cannot be doubted that a certain 

 and definite correspondence must exist between 

 the impression and the act it evokes. Indeed, 

 among lower forms of animal life this correspond- 

 ence is not only exceedingly well marked, but 

 constitutes in itself the sum total of the nervous 

 functions in such beings. But the highest ani- 

 mals, including man himself, may be said to ac- 

 quire a knowledge of their surroundings in an 

 exactly similar manner. When we talk of exer- 

 cising our senses — or when, to use a comprehen- 

 sive term, we speak of " feeling" — we are simply 

 expressing the idea of obtaining a certain knowl- 

 edge of our environments, and as a result we are 

 further enabled to act upon that knowledge in 

 ways and fashions relative thereto. 



Some such ideas as those just stated have 



