18 



NATURAL HISTORY. 



of the cell pass out through the tube into the tissues of the victim. It 

 of course takes many of these cells to produce paralysis, and after once 

 beino* used thev cannot return to their former condition. New ones are 

 being developed continually. 



These structures, called variously nettle-cells, thread- 

 cells, and lasso-cells, are very characteristic of the whole 

 group of Coelenterates ; and while those of the hydra are 

 too small, too few, or too weak to affect man, those of 

 some of the jelly-fishes are capable of producing considera- 

 ble pain. On our northern coasts there is but one jelly- 

 fish which has the reputation of nettling human beings. 

 This is the large blue-jelly (Cyanea), to be mentioned again 

 farther on. It would appear, however, that this form 

 differs in its nettling powers, or that different persons 

 Fl neuie^ens SC of ar hy d - are differently affected; for the writer, though he has 

 dra - often experimented with it, has never felt any of the 



stinging sensations which others describe. In more southern seas there 

 are several forms which nettle severely, the " Portuguese man-of-war " 

 being possibly the worst. Those who have been stung by it describe the 

 pain as very severe, and use such comparisons as "liquid tire" to describe 

 their sensations. 



The hydra is very simple in its structure. It consists merely of two 

 layers of cells, those of the inner layer being digestive, those of the outer 

 layer protective. There does not, however, seem to be much essential 

 difference between the two layers, for two naturalists have turned the 

 animals inside out, just as one would invert a bag, and then to all appear- 

 ances the former skin digested as well as the proper stomach layer. 



The hydra reproduces in two ways, — by eggs and by buds. In our 

 figure two of these buds are shown, the upper one being more advanced 

 than the lower. After the appearance of the tentacles in the bud a 

 mouth is formed, and soon after the young one separates from the parent. 

 It not infrequently occurs that a second individual will bud from the 

 young hydra before the separation is complete. 



The hydra is the type of a large group of animals called Hydroids, 

 which are especially abundant in the sea, where they present some of the 

 most astonishing features. Some of the marine forms are very much like 

 the fresh-water hydra, but others add features which make them, at least 

 in certain stages of their growth, greatly different. 



All along our northern Atlantic coast, near the low-water mark, one 

 can find quantities of snail-shells, each occupied by those most interesting 

 animals, hermit-crabs. On examining one of these shells one usually finds 



