18 INTRODUCTION TO ZOOLOGY. 



the sea, at a distance from the places where the parents 

 are fixed, and where they live and die. Were it not for 

 this wise arrangement, the locality would, in time, cease to 

 supply the conditions requisite for their existence, and the 

 species must perish. The young Tubularise use the tenta- 

 cula as feet, and, by their aid, remove themselves to a fitting 

 distance from the locality of the parent. 



The polypes of the third family (Sertulariadce*) resemble the 

 Hydra in shape, and are retractile within their cells. Their 

 common habitat or "polypidom"! assumes a tree-like aspect, 

 reminding us, in some species, of miniature ferns and other 

 vegetable productions. These are the corallines, whose fea- 



thery tufts decorate the ex- 

 terior of the common Oys- 

 ter or Mussel to which they 

 are frequently attached. 



The cells, numerous as 

 they are, are each inha- 

 bited by a polype, not as 

 a mere occupant of the 

 cell, and possessed of the 

 power of leaving it at plea- 

 sure, but forming, with the 

 cell, the stem, and the root, 

 one living mass. Each 

 polype is connected by a 

 thread with the medullary 

 matter in the centre of 

 each branch, and thus all 

 the parts are united into 

 a compound animal, fur- 

 nished with a multitude 

 of mouths; for each indi- 

 dual polype contributes, 

 by the food he takes, 

 to the nutriment of all. 



easily understood by the magnified respresentation of one 

 of these animals given in Fig. 9. The repetition of any 



* From sertula, a little nosegay, wreath, or chaplet of flowers. 

 t The term is applied to the horny sheath with which the soft body 

 of the polypes is invested. 



