HTDROZOA. 23 



foot, through which particles of an excrementitious 

 nature occasionally pass. 



By means of its adherent disc the Hydra at- 

 taches itself to submerged stones, plants, and the 

 surfaces of floating sticks or leaves. It is not, 

 however, permanently fixed, but has the power of 

 effecting changes in its position, either by the 

 slow gliding motion of its base, or the repetition 

 of certain leech-like movements, in which both 

 tentacles and disc take part. Occasionally, the 

 disc is protruded above the surface of the water, 

 and, thus acting as a float, ena])les its possessor to 

 remain, for a time, in this suspended condition. 



The reproductive organs of the Hydra do not 

 admit of being observed at all periods of the year, 

 seldom making their appearance before the ap- 

 proach of the cold weather of autumn. Their 

 position, however, is constant, the spermatozoa 

 being contained in conical processes of the body- 

 wall which arise close to the bases of the tentacles, 

 while the ova are enclosed in rounded elevations 

 of much greater size, and situated nearer the fixed 

 extremity (^fig. 3, d). Sometimes Hydrse are met 

 with in which only one set of reproductive ele- 

 ments can be detected. Not more than one of 

 the large protuberances, containing but a single 

 ovum, is usually developed at the same time, and 

 when two occur, they always arise from opposite 

 sides of the animal. The ovum, having pushed 

 itself through the body-wall, is seen to be invested 

 with a spherical envelope, brownish or rosy in 

 tint, and studded with a number of rough points, 

 which some writers describe as bristles. Mr. Han- 

 cock, however, more properly regards them as 



c 4 



