222 



THE AMERICAN MONTHLY 



[iJecember, 



new ones of his own. In 1744 Trem- 

 bley published the results of his ex- 

 tensive studies, and of this work it 

 has been well said by All man that it 

 ' marked out one of the great epochs 

 in the history of biological research.' 



As to classification, the genus Hy- 

 dra^ Linn., ranks only a little above 

 the Protozoa. It is the lowest form 

 of the great animal sub-kingdom 

 Ccslenterata (or Sac-animals), which 

 is divided into two classes, Hydi'ozoa 

 and Actinozoa, and Hydra is the 

 type of the former class, just as Ac- 

 tinia (the Sea-Anemone) is that of 

 the latter. Some 6 or 7 species of the 

 genus have been described, but only 

 3 of them, H. viridis^ H. V7dgaris^ 

 and H. fusca^ seem to be well 

 founded, the others being probably 

 only varieties. 



Hydra consists essentially of an 

 elongated nodular sac of protoplasmic 

 substance, imbedded in which are 

 found large numbers of colored 

 granules. At the upper end of this 

 sac is a simple opening, the mouth, 

 and just below this is a circle of 

 tentacles, usually from 6 to 10 in 

 number, (fig. 17). At the lower ex- 

 tremity the body is furnished with a 

 flattened suctorial disk, by means of 

 which the animal usually attaches 

 itself to filaments of algae, rootlets 

 of duck-weed, and similar objects, 

 while its slender tendril-like tentacles 

 are slowly and gracefully waving 

 about in search of prey. Sometimes 

 it uses this disk as a sort of float, and 

 hangs head downward, suspended 

 from the surface of the water, The 

 body and tentacles of Hydra ^ when 

 fully extended, seldom measure over 

 \ — ^ of an inch in length, except in 

 the case of the rare species, H.fiisca^ 

 where the animal sometimes attains 

 a length of several inches, owing to 

 the extraordinary development of the 

 tentacles, which,* in that species, are 

 many times the length of the body, 

 while in H. vulgaris they are but very 

 little longer, and in H. znridis usu- 

 ally somewhat shorter. The tentacles 

 are hollow, each being traversed by a 



canal communicating directly with 

 the body cavity. The body and ten- 

 tacles oi Hydra are possessed of most 

 remarkable extensile and contractile 

 powers. At one moment the animal 

 may be extended to such a degree 

 that the tentacles are almost invisible 

 by reason of their fineness ; when, 

 upon being disturbed, it instantly 

 contracts until it appears like a minute 

 jelly-like lump, studded with a few 

 stubby knobs. It then slowly ex- 

 pands until it is again fully extended. 

 While it usually remains attached to 

 the same object for a long time, it has 

 the power of changing its position, 

 either by a leech-like crawling move- 

 ment, or by. floating passively in the 

 water until it comes in contact with 

 some other object to which it wishes 

 to adhere. 



Hydra is extremely voracious. It 

 subsists entirely upon animal food, 

 consisting mainly of minute worms 

 and the smaller entomostraca. When 

 the prey has been caught by means 

 of the tentacles extended for that pur- 

 pose, these contract, and the unfor- 

 tunate victim is forced with remark- 

 able violence into the digestive cavity 

 of the polype, the softer parts being 

 there absorbed, and the undigested 

 portions ejected through the mouth. 



With regard to the liistology of the 

 subject of our sketch, many very di- 

 verse views have been held. Ecker, 

 one of the first investigators of its 

 minute structure, came to the conclu- 

 sion that it was not composed of 

 cells, but of a sort of sarcode, or, as 

 he called it, ' unformed contractile 

 substance,' thus bringing it into close 

 structural affinity with the rhizopods. 

 But better optical appliances and im- 

 proved methods of research have am- 

 ply proven the fallacy of these views, 

 and it is now universally conceded 

 that Hydra is composed exclusively 

 of cells and cell-derivatives. The 

 most valuable researches on the sub- 

 ject are those of Kleinenberg, and to 

 his admirable monograph, published 

 in 1 87 3, I am indebted for much val- 

 uable information. 



