xxxviii THE VOYAGE OF H.M.S. CHALLENGED 



arc elsewhere separated by the hollow space of the gastrovascular system, whose bounding 

 surfaces are only fused together at definite points. The exumbrella is the free convex 

 surface of the thick dorsal wall ; the subumbrella is the free concave surface of the thin 

 ventral wall. 



§ 59. Umbrella dorsalis (" notumbrella," upper or dorsal umbrella). In the more 

 limited sense this part is usually simply termed the " umbrella," as it forms the principal 

 mass of the umbrella, and as its voluminous gelatinous disk is much thicker than the thin 

 gelatinous plate of the ventral umbrella. Its upper convex surface, covered with dorsal 

 ectoderm, is the "exumbrella" (c). Its lower concave surface forms the outer or 

 abaxial wall (umbral wall) of the gastrovascular system, and is covered by its flat " dorsal 

 endoderm." The two epithelial layers of the dorsal umbrella, the outer ectodermal layer 

 and the inner endodermal, are separated by the powerful mass of the gelatinous body 

 (" collosoma "). They never run into one another, as they pass immediately at the 

 umbrella margin into the two corresponding epithelial plates of the ventral umbrella. 

 The endodermal epithelium of the dorsal umbrella consists of flagellate cells of an 

 indifferent nature, whilst its ectodermal epithelium often forms thread cells, more rarely 

 also epithelial muscular cells (" exumbral muscles "). 



§ 60. Exumbrella. The convex outer surface of the dorsal umbrella, which we call 

 shortly the " exumbrella " in many Medusa?, is perfectly smooth, arched equally without 

 any special characteristic, and covered uniformly by the simple ectodermal epithelium. In 

 many other Medusa?, on the contrary, it is distinguished by repeated projections in the 

 form of nodes, ribs, ridges, spicules, &c. These projections are often distinguished by 

 accumulations of thread cells, often also of pigment cells, and therefore serve as weapons of 

 defence of the umbrella. Projecting radial urticating ribs are found among the Craspe- 

 dota? that is in many Anthomedusa?, e.g., four perradial in several Codonida? and Tiarida?, 

 eight adradial in Ectopleura and Ctenaria (System, fcaf. vii. fig. 7), sixteen in Pec- 

 tyllis (Pis. III., IV.), Pectanthis (Pis. VII., VIII.), and Tesscrantha (PI. XV). In 

 Corynetes the whole exumbrella is overspun with a network of ridges, having projecting 

 urticating papilla? at their points of junction ; more commonly the urticating papillae 

 are scattered equally over it (e.g., Thamnostylus, PI. I.). Among the Acraspeda?, 

 such projecting urticating ribs are of less morphological importance than the deep furrows 

 of the exumbrella, by which the latter is divided into a number of gelatinous plates. 

 Very often there is a deep and distinct circular furrow or coronal furrow, which separates 

 the central umbrella disk from the peripheric umbrella corona. Whilst the former is usually 

 smooth, the latter, on the contrary, is often divided by radial furrows into projecting "gela- 

 tinous sockels or pedalia," which serve to bear the tentacles and rhopalia, as e.g., in Peri- 

 phylla (Pis. XVIIL, XIX.), Nav.phanta (Pis. XXVII., XXVIIL), and Atolla (PI. XXIX.). 

 The exumbrella of C&phea (System, taf. xxxii.) bears large, conical urticating papilla?. 



§ Gl. Umbrella ventralis (" eoelumbrella," lower umbrella or ventral umbrella). 



