9C> 



HYDEOZOA. 



of the bell-shaped polyp (3), but no tentacula are yet distinguishable ; 

 a rudimentary septum becomes visible, stretching across the bottom of 

 the cell, through the centre of which the granular matter, now collected 

 into a mass occupying only a portion of the stem, is seen to pass. The 

 polyp and cell gradually grow more denned (4, 5, 6), and the tentacula 

 become distinguishable ; the cell, moreover, is seen to be continued in- 

 wards by a membranous infundibular prolongation of its margin (V), 

 that immediately reminds us of the funnel-shaped membrane of Tubi- 

 pora ( 157), and its office is no doubt similar. As the development 

 proceeds, the tentacles become more perfect (8,9), and the polyp at 

 length rises from its cell to exercise the functions to which it is 



destined. 



Fig. 46. 



Diagram illustrating the mode of growth of a Sertularian polypidom. 



(229.) The main feature that distinguishes the Sertularian Zoophytes 

 from the Hydrce seems to consist in the fact that, whereas in the latter 

 each newly-formed offset becomes detached from the parent stock and 

 enjoys a separate existence, in the former each new sprout remains 

 permanently adherent, the successive generations being united into a 

 ramified stem, which is common to the entire group. The Hydra, 

 having no polypary or outer covering, when it dies, entirely perishes : 

 but in the Sertularida3, every sprout, when it dies, leaves its horny in- 

 tegument attached to the general community ; and thus, in time, there 

 results an elaborately-branched stem, the complexity of which increases 

 with the age of the colony. 



