SECT. IV PHYLUM OCBLENTERATA 131 



narrow or proximal end, it is produced inwards into a sort of 

 circular shelf (sit), perforated in the centre : upon this the base of 

 the polype rests, and through the aperture it is continuous with the 

 common stem. When irritated — by a touch or by the addition of 

 alcohol or other poison — the polype undergoes a very marked con- 

 traction : it suddenly withdraws itself more or less completely into 

 the theca, and the tentacles become greatly shortened and curved 

 over the manubrium (P. B). 



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 homy consistency, the perisare (p), and 

 an inner, delicate, granular layer, the ccenosarc (cos), 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, as we shall see, to the 

 action of cilia. At the base of each zooid or branch the perisare 

 presents several annular constrictions, giving it a ringed appear- 

 ance : for the most part it is separated by an interval from the 

 ccenosarc, but processes of the latter extend outwards to it at 

 irregular intervals, and in the undeveloped zooids (Btl. S) the two 

 layers are in close apposition. 



In the blastostyle both mouth and tentacles are absent, the 

 zooid ending distally in a flattened disc : the hydrotheca of a 

 polype is represented by the gonotheca (g.th), 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 surface. They are ulti- 

 mately 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 a polype (Fig, 96) 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 (ect) and the endoderm (e7id) : 

 between them is a very delicate transparent membrane, the 

 mesoglcea or supporting lamella (msgl), which, unlike the inter- 

 mediate layer of sponges, contains no cells and is practically 

 structureless. The same three layers occur in the manubrium, 

 the ectoderm and endoderm being continuous with one another at 

 the margin of the mouth. The tentacles are formed of an outer 

 layer of ectoderm, then a layer of mesoglcea, and finally a solid 

 core of large endoderm cells arranged in a single series. The 



K 2 



