HYDROIDA ; SERTULARIAN POLYPES. 527 



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 resemblance is still more striking 

 when the mode of propagation in the two groups is compared. 



1178. 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 pur- 

 pose. 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 vesbles 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). In some instances the embryo before quitting 

 the ovarial vesicles acquires exactly the form of an exceedingly 

 minute Medusa. 



1179. Besides the Hydne and Sertularia3 there is a third type 

 of Hydroid Polypes, the Tubularics. In these, which, like the 

 Sertularian Polypes, are for the most part compound animals, the 

 polypidom is usually of a soft and membranous texture, some- 

 times quite rudimentary or altogether wanting ; and the Polypes 



larian Polypes ; and hence thjey 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. 



