6 ZOOPHYTES. 



under the best conditions, no trivial embarrassments impede the artificial 

 conservation of delicate creatures abounding in the sea. The water, 

 also, sometimes contracts a noxious principle, from causes eluding conjec- 

 ture and observation. But it may be fit to warn the naturalist against 

 employing tall vessels, with copious vacuities above the water, on all occa- 

 sions ; for, if the air, vitiated by respiration, escaping from the surface, be 

 re-absorbed, it cannot be otherwise than deleterious. This is well illus- 

 trated by attempting to keep fishes in deep vessels, with a scanty propor- 

 tion of water, to prevent their leaping over the side. They generally 

 perish in a short time ; and hence shallower vessels nearly full are prefer- 

 able. 



Five successive heads have vegetated from the same stem of a speci- 

 men of this Tubularia in captivity, without computing those necessarily 

 borne, as the medium of its previous prolongation. But it is not to be 

 inferred that the reproductive power was then exhausted, for one so pro- 

 lific, in the brief period devoted to observation, may regenerate ten or 

 twelve times during the course of its existence. 



Some remarkable facts attend renewal of the head ; and first, the 

 prolongation of the stem seems absolutely dependent upon it. Having lost 

 its head, the stem to all appearance remains stationary, unless in the wound 

 closing ; but from the moment that the rising internal bud reaches the 

 vacant extremity, in its integument, the neck, or that portion sustaining 

 this young hydra, visibly lengthens, and so continues, until further pro- 

 longation is arrested, by the separation and fall of the regenerated parts. 

 The wound cicatrizes again. If reproduction follow, by another embryo 

 rising within to issue from the summit, a new prolongation ensues also ; 

 and so on with a third, a fourth, or more. Thus are formed as many nodes 

 or articulations of the stem. 



Prolongation of the stalk seems combined with the evolution of the 

 hydra by one of the few invariable laws ascertained. But the irregular 

 duration of the successive hydrse or heads, produces an irregularity in the 

 accessions to the length of the stalk. One shoot extending six or eight 

 lines may be followed by another of only two or three ; and the prolonga- 

 tion seems scarcely sensible where the head flourishes merely to decay. 



