HYDRAFORM OR SERTULARIAN POLYPES. 491 



therefore, that by the older Naturalists, who were ignorant of the 

 existence of the Polypes, these productions were regarded as of 

 a vegetable nature, and were termed Sea-Mosses. The resem- 

 blance is still more striking when the mode of propagation in 

 the two groups is contrasted. 



1055. Although the reproductive gemmules are in some 

 instances produced from the Polypes themselves, as in the 

 Hydra, a more special apparatus is usually evolved for the 

 purpose. At certain periods, there are formed from particular 

 spots upon the stem of the Sertularia and its allies, expansions of 

 its horny structure, somewhat resembling those which encase 

 the Polypes, but usually larger. These ovarial vesicles, which 

 so much resemble the urns of Mosses (VEGET. PHYSIOL. 429), 

 are like them provided with a lid, which falls off when the con- 

 tained gemmules are mature, so as to permit their escape ; and 

 after their purpose is thus completed, the vesicles fall off, like 

 the seed-capsules of all plants. The gemmules are usually 

 clustered around a central column (analogous to the columella of 

 Mosses) ; and when mature they swim forth by the action of 

 the cilia with which they are provided, being detached from the 

 central column at the same period that the lid of the vesicle falls 

 off. The gemmules move to and fro by the vibration of their 

 cilia, during a period which varies from a few hours to two or 

 three days. When they have fallen upon a site fit for their 

 development, they attach themselves to it by a root-like fibre, 

 and then begin the formation of the polypidom. The real 

 nature of this gemmule has been elsewhere explained (ANIM. 

 PHYSIOL. 745). 



1056. All the compound Hydroida are inhabitants of the 

 ocean. Some of them seem to attach themselves indiscriminately 

 to any solid mass ; whilst others seem to have a preference for 

 some particular kind of support. Thus one is found only on 

 rocks which are constantly beneath the surface ; others on those 

 which are occasionally uncovered by the sea ; and others attach 



larian Polypes ; and hence they are associated together in the minds of those 

 ignorant of this department of Natural History. The real Corallines may be 

 distinguished by the absence of any trace of cells upon their surface. 



