60 MICROSCOPICAL STUDIES. 



sustenance only, but equally for the whole colony, for the base of the 

 body of each individual passes into a tubular prolongation, cCBnosarc, 

 continuous with a similar tubular tissue in the main stem. If one 

 polypite capture food, part of the products of digestion pass into the 

 tubular coenosarc, and is conveyed to neighbouring parts of the 

 colony for purposes of nutrition. And upon this unselfish mutual 

 help, depends the power of the colony to set free from the purely 

 nutritive (vegetative) function, certain individuals or zooids, charged 

 specially with the reproduction of the species. These specialized 

 zooids are developed in the axils of the branches and are elegant 

 elongated urns in shape, gonothecae (g), wherein the coenosarc breaks 

 up into granular bud-shaped masses that gradually evolve into 

 transparent delicate swimming-discs. Their gracefulness has to be 

 seen to be understood. One has to watch them as, so many streamer- 

 decked bells, they sink slowly through the water, of a sudden to 

 regain activity and pass upwards by a series of vigorous jerking 

 pulsations, to cease after a number of strokes and become inert, 

 slowly-sinking parachutes once more. 



Medusae are generally bell-shaped, with the true edge turned hori- 

 zontally inwards to form the velum (Fig. 5, v.)> but in the form under 

 notice, the shape is nearly that of a flattened disc, and the velum in 

 consequence of this difference is all but absent. From the apparent 

 edge of tne disc proceed a number of tentacles beset with stinging 

 cells, while from the centre depends a tubular process, the manubrium 

 (m.), having the mouth at the free extremity and a slightly dilated 

 cavity, the atrium (a.), at the attached base. From the atrium proceed 

 four radial canals (re), while around the edge of the disc, just inside 

 the ring of tentacles, is a circumferential canal (c.c.), into which the 

 radial canals empty. All these cavities are lined with ciliated endo- 

 derm, which shows little divergence in form in the various regions. 

 The ectoderm forming the exterior coat of the Medusa exhibits much 

 more differentiation ; while on the upper surface it consists of flat- 

 tened cells, on the under it developes radially disposed muscle fibres, 

 and on the manubrium, longitudinally disposed ones. Between the 

 ectoderm of the upper surface of the disc, and the endoderm of the atrium 

 and radial canals, is the intermediate body layer, the mesoglsea. In 

 Obelia, it is thin, but in some species it is often enormously developed 

 as a thick jelly-like substance (Aurelia, Pelagia, &c.), that gives 

 cause to the popular name of the Medusae. The term umbrella (u.), is 

 bestowed upon this layer. The sub-umbrella (s.u.), another layer of the 

 same tissue, but quite thin in even the best developed instances, is 

 found between the endoderm and the lower surface of the animal. 

 A point sometimes overlooked is that the radial canals are joined 

 together by a horizontal layer of endoderm cells, so that where the 



