POLYPI. 123 



this tribe that approaches the nearest in its character to the 

 sponge, which it resembles in the nature of its internal tex- 

 ture. Each of the polypes with which its surface is studded 

 has eight serrated tentacula. Fig. 57 represents one of these 

 polypes detached. Polypes may thus be united in immense 

 numbers in one mass, having mutual organic connexion. In 

 other cases they may form smaller clusters, or be even to- 

 tally unconnected. Sometimes the detached polypes are 

 still disposed to assemble in groups, as is the case with the 

 Zoanthus of Cuvier* (Fig. 5S:) at other times they are al- 

 together isolated, as in the Hydra viridis (Fig. 59.) 



Polypi form a very extensive order of zoophytes, abound- 

 ing in every part of the ocean, but growing in greatest lux- 

 uriance in the warmer regions of the globe. Their flesh 

 exhibits the same granular appearance as that of the spon«rc, 

 but it is generally firmer, and often intermingled with masses 

 of calcareous matter. The tentacula, which may be com- 

 pared to arms, vary in number and in length in different 

 species of polypi, and sometimes, instead of a single row, 

 each of the mouths has two or more series of tentacula placed 

 around it. They are formed of a prolongation of the soft 

 substance of the polypus, and are sometimes tubular; and 

 their cavities are then continuous w^th that of the general 

 internal cavity into which the several mouths open. Be- 

 sides being flexible in every direction, the tentacula are also 

 capable of being lengthened or shortened at the pleasure of 

 the animal. Their elongation is produced by the propulsion 

 of a fluid into their interior, derived from the general cavity 

 of the body; and their retraction is effected by the return of 

 the same fluid. 



The whole arrangement of the tentacula on the margin of 

 the projecting mouths bears a striking resemblance to a 

 flower, especially to those which, like the daisy, or china- 

 aster, have the corolla composed of slender radiating petals. 

 We find, indeed, that as the organs of zoophytes become 

 more developed, the alfmities which these lower departments 



• The Hydra sociata of Gmclln; the Acl'mia sociata of Kills. 



