§ 8. THE HYDRA, OR FRESHWATER POLYPE. 599 



these muscles in some of the larger Infusoria.* The 

 number of the cilia, even in an animalcule invisible to the 

 naked eye, is almost incredible ; Dr. Grant calculates that 

 a single polype of a Flustra has 400 millions. 



8. The Hydra, or freshwater Polype. f — Before 

 describing the zoophytes which are the immediate subject of 

 this lecture, I would call your attention to one of the most 

 simple forms of animal life, that abounds everywhere in 

 freshwater streams, and being relatively of considerable 

 size, will afford a convenient illustration of some of the 

 vital phenomena exhibited in the coral zoophytes that 

 give rise to reefs and islands in the seas of warm climates. 

 This is the Hydra, or freshwater polype, of which 

 several kinds inhabit our ponds, rivulets, &c. They 

 are generally attached to the stems and leaves of aquatic 

 plants ; and the largest species when in an expanded state 

 is from a quarter to half an inch long, and of the size 

 of a hog's bristle ; it is constricted at the end attached to 

 the plant, and has an aperture or mouth at the free 

 extremity, from around which proceed from six to ten 

 long delicate tentacula (Lign. 132, Jig. 4). The Hydras 

 present an example of a highly endowed organism of the 

 simplest structure ; the whole animal consisting merely of 

 a gelatinous, transparent, open cylinder, or tube, contracted 

 at one extremity, and having the margin of the other pro- 

 longed into tubular tentacula. It is, in fact, a stomach, or 

 digestive sac, with no appendage but the instruments for 

 seizing its prey. A vertical section {Lign. 132, Jig. 5) 

 highly magnified, shows the interior of the receptacle for 

 the food, the relative thickness of its substance, and the 



* See the representation of the Cilia on the rotatory organs of the 

 Kotifer or Wheel-animalcule in " Thoughts on Animalcules/' p. 35. 



t Polype, or polypus (many feet), is a name derived from the tenta- 

 cula, or processes which in some species serve for progression, in others 

 for respiration. 



