HYDRA. 79 



tract quickly, on removal to the light, and always as if to evade some 

 painful impression. 



Probably adhesion is spontaneous, as with the polypus of the fresh- 

 waters and the Actinia of the seas. But the Hydra tuba commonly re- 

 mains stationary where it has taken a position ; if affixing first to the bot- 

 tom of a vessel, it continues permanently there. If dropping from its 

 place when affixed to the side, or should the water be repeatedly agi- 

 tated while either adults or young are loose, they seldom adhere after- 

 wards ; nor does the animal fix readily at any time. 



Its trumpet form is sometimes entirely lost, and the figure of a hand 

 bell assumed by the upper portion of the body, near the fixture of the 

 apex, relaxing like a coarse thread, extending two lines or more. Then 

 the remainder of its pendent body flattens in a campanulate form. When 

 several of a group assume this figure, inexperienced observers might be 

 deluded by the change.— Plate XIV. fig. 26; XX. fig. 19, a. 



A locomotive faculty, though rarely exercised, and only in the lowest 

 degree, is undoubtedly enjoyed by these creatures. The adult is never 

 seen in the course of progression. Indeed, I doubt if its advance would 

 be perceptible ; but the young hydra withdraws unnoticed from the parent : 

 and in event of successive or of a series of generations, the whole indivi- 

 duals constitute a colony dispersed around its original founder. Their 

 segregation generally tends upwards. 



Keeping this latter fact in view, it will be discovered that the like 

 general principle governs the compound zoophytic tribes : the increment 

 of the Sertularia is by ascending from the original cell giving birth to 

 the whole. 



Spite of advancing multiplication, numerous colonies of the hydra 

 remain concentrated within narrow bounds. Though some adults seem 

 almost constantly rivetted to the same spot, it appears essential that the 

 young germinating from the parent's side shall be capable of removing, to 

 leave room for more, as propagation is incessantly going on. After ad- 

 vancing in one direction, they sometimes recede a little, slowly and 

 imperceptibly. 



Thus the perceptions and active faculties of these creatures are ex- 



