REPORT ON THE DEEP-SEA MEDUSJB. xlv 



(as, for example, at the umbrella margin). The indicating weapons (" arma urticaria ") 

 appear under very varied forms as roundish urticating knobs (in the whole ectodermal 

 surface), closed urticating rings (in the outer wall of the tentacles), narrow urticating 

 streaks or flat urticating pads (at the umbrella margin), conical urticating papillae (in the 

 exumbrella and subumbrella), composite urticating clubs and urticating batteries (at the 

 end of the tentacles), and so forth. All these urticating weapons consist of epithelial 

 accumulations of numerous urticating cells, which usually lie compacted in the upper 

 surface of the ectoderm, and which throw the urticating threads and fluid from 

 their thread cells, when their freely projecting urticating bristle (" cnidocilium ") is 

 touched. They are usually developed in a subepithelial layer, the " interstitial tissue." 

 As soon as the thread cells and their filaments are fully developed in the cnidoblast they 

 become erect, and pass from the subepithelial into the superficial epithelial layer. 

 "When this is very thin and flat {e.g. in the exumbrella) the thread cells originate in the 

 epithelial cells of the upper surface itself. In many places, principally on the umbrella 

 margin, the thread cells lose their original significance as weapons, accumulate thickly 

 compacted in firm masses, and so assume the function of a supporting skeleton. Such 

 subepithelial urticating skeletons (" sceleta urticaria ") attain a high development in the 

 Trachylinae (Trachomedusae and Narcomedusae). They sometimes form a firm urticating 

 ring on the umbrella margin (on the distal margin of the coronal canal (Pis. IX.-XIV. 

 nc), sometimes radial urticating streaks, which run centripetally from the urticating ring 

 and rise upwards in the exumbrella. These centripetal urticating streaks serve as firm, 

 elastic support, sometimes for the freely projecting auditory clubs (auditory clasps " oto- 

 porpas," Pis. IX.-XIV. oo), sometimes for the dorsally inserted tentacles, whose bases they 

 connect with the umbrella margin (umbrella clasps, " peronia," Pis. IX.-XIV. en; comp. 

 § 68). As the cnidoblasts in these supporting shields are accumulated in a number of 

 layers, the one above the other, and lie deep under the epithelial upper surface, the en- 

 closed filaments, which are no longer able to escape, lose their function as armature, 

 whilst the hard nematocysts which assume the supportive function of the firm and elastic 

 cartilaginous tissue become proportionately more strongly developed (PL XIV. fig. 

 12, en). 



§72. Nervous system. In all Medusas the nervous system stands at a very low 

 stage of development, as it retains the most immediate connection with its place of de- 

 velopment, the ectodermal epithelium, and as neither its central nor its peripheric parts 

 have become completely and independently separated.- We can usually distinguish in 

 all Medusae a central and a peripheric section of the nervous system. The circular 

 central part lies either on the umbrella margin or above it on the subumbrella ; whilst 

 the peripheric part extends chiefly on the subumbrella in the form of a diffuse nervous 

 plexus. Both in the central and peripheric part we find smaller and larger ganglion cells, 

 mixed with finer and coarser fibrillar (PL XIV. figs. 9, 10). These cells are most closely 



