170 MICROSCOPIC FAUNA. 



nodular, or exhibits spiral or other raised lines, and it contains 

 numerous vacuoles within. And if a Hydra be crushed between 

 glasses, the substance breaks up into globular, minute masses like 

 cells, and which often continue expanding and contracting like 

 amoeba. Imbedded in the superficial portions of its substance, 

 and more especially in the tentacles, are certain curious bodies 

 called the stinging-organs. They consist of an oval, truncate, 

 firm capsule of comparatively considerable thickness, marked by a 

 double outline ; within this capsule is contained a very long and 

 slender filament. I have seen it extend to about ten times the 

 length of the capsule. It is apparently of great strength, though 

 of such tenuity as to be almost invisible. At the base of this 

 filament are four minute spines. In the undisturbed state of the 

 Hydra this filament is coiled up along with the spines in the 

 capsule; but if the creature is squeezed or touched or heated 

 slightly, the spines with this filament fly out with great velocity. 

 From the fact that formic acid appears in the capsules, these 

 spines seem to possess a stinging power ; and this may account for 

 the apparent numbness which some have perceived in those Ento- 

 mostraca which have been pierced by them. I have seen an 

 Entomostracon which was pierced by one of these darts and held 

 captive by the slender filament, apparently waiting until the 

 filament should be drawn in, and one of the tentacles stretched 

 forth to enwrap the prey. For the prey of the Hydra consists of 

 Entomostraca, small Annulata, etc., and very rapacious it is ; but 

 in what manner it knows where its prey is, while at a distance, so 

 as to dart its harpoon and pierce it, is a mystery, since no trace of 

 eye or eye-spot can be seen. 



There are few facts more extraordinary than the strange 

 power which the Hydra has of reproducing its lost parts; indeed the 

 power goes much further than mere reproduction. If a Hydra is 

 w^ell fed and in vigorous condition, it may be turned inside out, 

 since its body is merely a hollow tube ; it may be cut transversely 

 or longitudinally, and will still outlive the process, and the 

 excised parts will be reproduced, the internal surface will change 

 places with the external, and the creature apparently suffer but 

 small inconvenience. I have said that in order to prosecute these 

 and other experiments with Hydra, they should be in thorough 



