REPORT ON THE DEEP-SEA MEDI7SJE. xxxix 



This part of the umbrella is usually simply termed umbrella iu the wider sense, though 

 this name really belongs only to its lower concave surface, which is covered by ectoderm. 

 Its upper, convex surface forms the inner or axial wall (subumbral wall) of the gastro- 

 vascular system, and is covered by its high " ventral endoderm." The two epithelial 

 layers of the ventral umbrella, the ectoderm of the concave surface (subumbrella), and the 

 endoderm of the convex surface, only pass into one another at the umbrella margin, and 

 are likewise separated by a thin but firm supporting plate (210). This fulcral lamella of 

 the subumbrella is equivalent to the thick gelatinous body of the dorsal umbrella, 

 though much thinner, and passes immediately into the umbrella margin at the edge of the 

 latter. The endodermal epithelium of the ventral umbrella consists of high flagellate 

 cells, which also often form glandular cells, whilst its ectodermal epithelium (the " sub- 

 umbrella " in the more restricted sense) originates the most important part of the mus- 

 cular system of the Medusas (" subumbral muscles"). 



§ 62. Subumbrella. The convex inner surface of the ventral umbrella, which we 

 designate shortly as the " subumbrella " (in the more restricted and special sense), is of 

 special importance as bearing the muscular system, which affects the swimming motions 

 of the Medusas. It is, moreover, distinguished by varied differentiations of the ectodermal 

 epithelium, lining the umbrella cavity enclosed by it. Thus, for example, glandular cells, 

 pigment cells, and thread cells are often found disposed in a certain order on its ectoderm, 

 and in all Craspedotas it also forms the point of origin of the reproductive glands. 

 Whilst in most of the smaller Medusas (for example, the Craspedotas) the subumbrella 

 appears smooth and regularly vaulted, in most of the larger Medusas (chiefly Acraspedas) 

 it is folded repeatedly and distinguished by special projections. Among the more im- 

 portant of these are the gelatinous ridges which serve for the wider superficial extension 

 of the subumbral muscular system. They usually run in concentric rings {e.g., on the 

 coronal muscle of the Peromedusas (Pis. XIX., XXII.), more rarely in radial bunches (e.g., 

 Drymonema (Pis. XXX., XXXI. ). The mesenteries of many Anthomedusas, and Tracho- 

 medusas, Stauromedusas, and Cubomedusas may be mentioned as special processes of the 

 subumbrella, which project into the umbrella cavity in the form of vertical radial septa ; 

 we shall speak of them further on in the "umbrella cavity" along with the various 

 secondary cavities and niches, which penetrate from the umbrella cavity into the sub- 

 umbrella (§§91, 94). 



§ 63. Central and peripheric umbrella (" discus centralis " and " corona peripherica "). 

 In all Medusas a certain morphological and physiological contrast can be recognised more 

 or less distinctly between the central and the peripheric part of the umbrella ; the most 

 important part of the vegetative gastrovascular system lies in the former, the most 

 important part of the animal neurodermal system in the latter. We term the central 

 principal section of the umbrella, enclosing the stomach and mouth along with the oral 

 organs, the umbrella disk (" discus umbralis "), the peripheric principal section containing 



