POLYZOA. 3 



and down the other of each tentacle. The united 

 effect of these currents as a whole is a powerful 

 vortex, the centre of which is the mouth at the 

 bottom of the belt of tentacles ; and thus floating 

 particles of food, or living animalcules, are drawn 

 into the whirlpool, and presently engulphed in the 

 yawning gullet below. 



When alarmed, the animal contracts its muscular 

 threads and retires within its cell, the protrusile 

 membrane being drawn inwards, and the tentacles 

 closing into a compact bundle as they descend. 



Though there is little diversity in the form or 

 structure of the animals themselves in this Class, 

 there is much difference in the form, arrangement, 

 and composition of the cells. In general the 

 form is ovate or oblong; but this general shape 

 is variously modified, being tubular, club-shaped, 

 horn-shaped, cradle-shaped, square, three-sided, 

 rhomboidal, &c., &c. The arrangement is often 

 shrub-like; but when so, the branches may be 

 formed by a single series of cells, or of two or 

 more set side by side, or back to back, or both. 

 At other times the branches are creeping and 

 adherent, as well as the root-thread ; or the cells 

 may be arranged in close series without branch or 

 root-thread, either adhering in irregular patches, 

 as the Lepralice, or rising into broad flexible 

 leaves, as the Flustra, or in solid stony walls as 

 the Eschara. The cells may be horny or mem- 

 branous, with the calcareous element not deposited, 

 as the Vesiculariadce ; or they may be sunk in a 

 common fleshy or cartilaginous mass, as the Alcyo- 

 nidiada. Finally, the cell may be wanting, or, at 

 least, inseparable from the skin, as in the genus 

 Pedicellina. 



B2 



