MECHANISM OF EXPANSION AND RETRACTION. 413 



rolling outwards from around the ciliated polyp-like 

 arms, until the latter have risen entirely above the top 

 of the cell, when, the act of protrusion being com- 

 pleted, the tentacula separate and expand, and the 

 cilia upon their surface commence their rapid and 

 apparently voluntary vibrations, 



" Tinged with rays of infinite colour," 



and appearing to the eye, beguiled by the velocity 

 and regularity of their action, like strings of orient 

 pearls. 



The mechanism by which the protrusion and re- 

 traction of the polyp are effected, is somewhat com- 

 plicated ; and these movements are usually performed 

 with such rapidity, especially that of retraction, that 

 it is only by watching the animals perseveringly for 

 several hours together, and sketching down each step 

 of the process, whenever a momentary glance of one 

 of them in a favourable position can be obtained, that 

 any satisfactory idea of the precise mode of performing 

 these operations can be realized. Under propitious 

 circumstances, it will be seen that a very elaborate 

 muscular apparatus is provided for the purpose, con- 

 sisting of numerous bundles of delicate thread-like 

 cords arising from the bottom of the cell, and im- 

 planted around the basis of the tentacular apparatus. 

 No sooner has the little Polyzoon fully expanded 

 itself, than a very lively scene commences the neigh- 

 bouring visible particles floating in the surrounding 

 water are at once set in rapid motion, as though 

 suddenly brought within the influence of a mighty 

 whirlpool, terrible as the Maelstrom itself, and woe- 



