iv PHYLUM OELENTERATA 95 



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

 flickering movement may be observed, due to the presence 

 of vibrating cilia on the inner surfaces of the cells. In the 

 blastostyle both mouth and tentacles are absent, the zooid 

 ending distally in a flattened disc ; the hydrotheca of the 

 polype is represented by the gonotheca (g.th.), which is a 

 cylindrical capsule enclosing the whole structure, but ulti- 

 mately becoming ruptured at its distal end to allow of the 

 escape of the medusa-buds. These latter are, in the young 

 condition, mere hollow offshoots of the blastostyle : when 

 fully developed they have the appearance of saucers attached 

 by the middle of the convex surface to the blastostyle, pro- 

 duced at the edge into sixteen very short tentacles, and 

 having a blunt process, the manubrium, projecting from the 

 centre of the concave surface. They are ultimately set free 

 through the aperture in the gonotheca as little medusae or 

 jelly-fish (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 mesogltza 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 

 no cell-structure, but only a delicate lamination. It is, in 

 fact, not a cellular membrane or epithelium, like the ecto- 

 derm and endoderm, but a cuticle, formed, layer by layer, 

 as a secretion from the ectoderm cells (see p. 67). It is 

 of chitinoid or horn-like consistency, and, like the lorica 

 of many Protozoa, serves as a protective external skeleton. 



Embedded in the ectoderm are numerous clear, ovoid 



