REPORT ON THE DEEP-SEA MEDUSAE. xxix 



ectodermal epithelium, which pass into one another at the umbrella margin, the dorsal and 

 the ventral chrotal epithelium, or the " exumbrella " and " subumbrella." Both are distin- 

 guished in the same way as the dorsal and ventral endoderm. The exumbrella or the 

 dorsal ectoderm (also termed " chrotal epithelium of the notumbrella," or shortly " upper 

 ectoderm ") covers the whole convex surface of the gelatinous umbrella in the form of a 

 delicate, flat epithelium of very uniform and indifferent character. The cells of this 

 epithelium are usually extremely thin, but very extensible polygonal plates, which lie 

 immediately on the gelatinous body, and are covered on their upper surface by a very 

 delicate cuticule ; this often looks dotted or granulated, as at definite distances on it 

 there are thickenings in the form of nodules or small papillae. Vibrating flagellate cells are 

 wanting for the most part on the dorsal exumbrella, but they are often found on definite 

 limited spaces, especially on and near the umbrella margin ; the flagella are then usually 

 very delicate and fine. Pigment cells and urticating cells are more frequently found in 

 the exumbrella, especially in the vicinity of the umbrella margin and in the projecting 

 radial ribs, ridges, nodes, and papillse with which the convex outer surface of the umbrella 

 is covered in many Medusae. The subumbrella or ventral ectoderm (also termed the 

 " chrotal epithelium of the coelumbrella," or shortly the " lower ectoderm ") covers the 

 entire concave surface of the gelatinous umbrella from the oral margin to the umbrella 

 margin (in the Craspedotae to the free margin of the velum, in the Acraspedae to the free 

 margin of the lobe-corona or of the velarium). It shows a much more complicated and 

 varied nature than the dorsal ectoderm. Its cells are usually higher, more cubical, partly 

 covered by a cuticle, partly not covered. Part of the cells bear vibrating flagella at 

 definite points, and feeling bristles at others. In the same way, urticating cells, 

 glandular cells, and also often pigment cells are richly developed in many regions. The 

 differentiation of the ectodermal epithelium is most varied and important at the actual 

 margin of the umbrella, and its appendages, such as the tentacles, marginal lobes, and 

 organs which are developed there. Thus we can often separate a special " subepithelial 

 layer of cells " from the true epithelium (which only covers the free surface). As numbers 

 of thread-cells are often developed, a special " urticating tissue " (" tela urticaria ") often 

 arises, which, especially in the Trachomedusa? and Narcomedusae, forms a thick " urticating 

 ring " and the " umbrella-clasps " (" peronia ") running out from it. The firm and thickly 

 accumulated nematocysts there lose their original function of protective weapons, and 

 attain the significance of a supporting dermal skeleton (urticating skeleton, § 71). 

 Another portion of the ventral ectoderm, and, in fact, that portion which covers the 

 subumbral wall of the gastrovascular system in the Craspedotae, furnishes a subepithelial 

 layer from which the reproductive cells, both male and female, originate in this section of 

 the Medusae. 



§ 47. The inner covering (" epithelium, endodermale," or " gastrale "). The inner 

 covering or gastral epithelium (also termed "endoderma" in the more limited sense), in all 



