THE HYDR^, OR POLYPES. 13 



have fallen into the water. When in an expanded state 

 it resembles a slender, semi-transparent cylinder or tube, 

 about the size of a hog's bristle, from one-quarter to 

 three-quarters of an inch in length, and which is con- 

 stricted at the end that is attached to the plant. At the 

 free extremity there is an aperture, or mouth, surrounded 

 by several long, delicate, tubular arms, or tentacula, vary- 

 ing in number in the different species from six to ten. 

 The polype, when contracted, appears hke a little ball 

 of jelly, scarcely one-tenth of an inch in diameter; and 

 the arms are then shrunk into small papillae, or eminences, 

 forming a zone round the upper part of the body, (pi. i, 

 fig. 5). 



There are four or five British species ; the three com- 

 monly met with around London are figured in Plate i. 



I. The Green Polype, {Hydra viridis, pi, i, figs. 1, 

 9). — This species is of a delicate green colour. Its body 

 is cylindrical, but gradually becomes more slender to- 

 wards the lower extremity. It has from six to ten 

 tentacula, or arms, which are not so long as the body. 



Fig. 1, represents three individuals attached to a 

 twig, as seen expanded in a phial of water. 



Fig. 9, shews a polype in four different positions, to 

 illustrate its mode of progression. The polype per- 

 forms locomotion by extending its arms and attaching 



