12 THOUGHTS ON ANI3IALCX^LES. 



III. 



THE HYDRA, OR FRESH-WATER POLYPE. 

 Plate I. 



With the view of facilitating the comprehension of the 

 vital phenomena that will come under our notice, when 

 we enter in detail on the natural history of the animal- 

 cules selected as the subjects of these illustrations, I 

 would first direct attention to the objects contained in 

 this phial of water: they are living fresh-water polypes, 

 or HydrcB, (pi. i.); animals that present considerable 

 analogy in their structure to many of the simplest forms 

 revealed by the microscope; and, being relatively of con- 

 siderable size, and abundant in ponds and rivulets, 

 they afford a convenient illustration of some of the most 

 interesting physiological problems involved in the study 

 of this division of animated nature. 



The Hydra* is common in almost every pool of fresh 

 water, and is generally found attached to the stems or 

 leaves of aquatic plants, or to twigs and branches that 



* Hydra: so named from the fabled water-serpent, whose numerous 

 heads were reproduced as often as cut off. 



