94 MANUAL OF ZOOLOGY sect. 



The various branches of the common stem show a very 

 obvious distinction into two layers : a transparent, tough, 

 outer membrane, of a yellowish colour and horny con- 

 sistency, the perisarc (p), and an inner, delicate, granular 

 layer, the ccenosarc {cm), continuous by a sort of neck or 

 constriction with the body of each hydranth. The ccenosarc 

 is hollow, its tubular cavity being continuous with the 

 cavities of the polypes, and containing a fluid in which a 

 flickering movement may be observed, due to the presence 

 of vibrating cilia. In the blastostyle both mouth and tenta- 

 cles are absent, the zooid ending distally in a flattened disc ; 

 the hydrotheca of the polype is represented by the gono- 

 theca {gth), which is a cylindrical capsule enclosing the 

 whole structure, but ultimately becoming ruptured at its 

 distal end to allow of the escape of the medusa-buds. 

 These latter are, in the young condition, mere hollow off- 

 shoots of the blastostyle : when fully developed they have 

 the appearance of saucers attached by the middle of the 

 convex surface to the blastostyle, produced at the edge into 

 sixteen very short tentacles, and having a blunt process, the 

 manubrium, projecting from the centre of the concave sur- 

 face. They are ultimately set free through the aperture in 

 the gonotheca as little medusae or jellyfish (B-D), which 

 will be described hereafter. 



The microscopical structure of Obelia reminds us, in its 

 general features, of that of such a simple sponge as Ascetta, 

 but with many characteristic differences. The body is 

 composed of two layers of cells, the ectoderm and the 

 endoderm, the latter ciliated ; between them is a very 

 delicate transparent membrane, the mesoglaa or supporting 

 lamella, which, unlike the intermediate layer of sponges, 

 contains no cells and is practically structureless. 



The perisarc or transparent outer layer of the stem shows 



