ORGANIZATION OP POLYPES. 117 



maintained, and unity given to the compound body. 

 In so large a class we must expect to find great dif- 

 ferences in organization ; some are much simpler in 

 structure than others ; some are free to move about 

 from place to place ; others and a greater number 

 are fixed, as by a root, to the surface of some object : 

 but all the animals of the group have soft and in- 

 articulate, bag-shaped bodies, furnished at the upper 

 extremity with a mouth, or opening, leading to the 

 stomach. The mouth is generally surrounded by one 

 or more circles of fleshy arms, or tentacula, which ex- 

 pand, like the rays of a star, and in many cases are 

 contractile, or capable, at the will of the animal, of 

 being drawn in from their greatest extension, and trans- 

 formed into mere fleshy, bud-like points. Tentacula, 

 which, when fully expanded, are (in the Hydra) several 

 inches in length, by a voluntary effort, and with great 

 rapidity, contract so as nearly to disappear altogether. 

 In many kinds the tentacula, however, are non-contrac- 

 tile, and are either constantly expanded in the water, or 

 merely drawn within the walls of the cell in which the 

 animal lives, without any diminution of their volume. 

 The Polypes possess no obvious nervous system. Their 

 respiration is supposed to be conducted by cilia, which 

 clothe the surface of the tentacula, and maintain a con- 

 stantly-changing current of water on the delicate sur- 

 face of those organs. 



While there is a great common resemblance between 

 the skeletons, or polypidoms, of all the compound Zoo- 

 phytes, the animals by whose organs they are secreted 

 are so different, that zoologists arrange them in two 



