148 THE MECHANICAL FUNCTIONS. 



their cavities are then continuous with that of the 

 general internal cavity into which the several 

 mouths open. Besides 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 pro- 

 pulsion of a fluid into their interior, derived from 

 the general cavity of the bodj^ ; 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 com- 

 posed of slender radiating petals. We find, indeed, 

 that as the organs of zoophytes become more deve- 

 loped, the affinities which these lower departments 

 of the animal kingdom retain with plants, are more 

 marked and more predominant. In the construc- 

 tion of zoophytes, nature seems still to keep in 

 view the models of vegetable forms, the characters 

 of which, while effecting the transition from one 

 kingdom to the other, she continues to impress on 

 her productions. Zoophytes, both in their outward 

 form, and in the disposition of their internal organs, 

 preserve the symmetrical arrangement round a 

 common centre so generally exhibited in plants, 

 and especially in flowers, and in the verti ciliated 

 leaves and branches.* Hence the radiated or star- 

 like forms which predominate in most of the 

 animals composing this class : and hence they 

 have obtained the title of Radiata, by which Cu- 

 vier has designated them. 



Like the animals of the sponge tribe, Polypi are 



* See page 78. 



