xxxiv THE VOYAGE OF H.M.S. CHALLENGER. 



the umbrella surface, whilst in many larger and higher Medusae the supporting 

 plate of the subumbrella is raised in concentric circular folds, which are covered by 

 corresponding folds of the muscular plate (e.g., Lucernaria, PL XVII. fig. 20 ; Peri- 

 phylla, PI. XXII. fig. 22). 



§ 53. Mesodermal muscular cells (" myoblasti mesodermales " " tela muscularis meso- 

 dermalis "). When the folded epithelial muscular leaves increase in extent and become 

 further developed, their growth is not limited to the formation of folds, but the epithelial 

 or subepithelial muscular cells emerge completely from their point of origin, the epithe- 

 lium, and form independent " mesodermal muscular cells." As they separate in great 

 quantities from the epithelium and become united to special plates or bundles, they pass 

 inwards in the connective tissue and form perfectly independent mesodermal muscles. 

 Such mesodermal muscles are more commonly found in the system of the longitudinal 

 muscles than of the circular muscles, more rarely and chiefly in the larger species among 

 the Craspedotse, but more frequently among the Acraspedas. Thus, for example, in the 

 large Peromedusas, the powerful deltoid muscles of the subumbrella, the longitudinal 

 muscles, and root muscles of the tentacles, &c. (Pis. XX.-XXIV. md, mh, &c), belong 

 to this category. Here the muscles are frequently detached so completely from the 

 epithelium that later on they are separated from it by a special supporting lamella or 

 even a thick gelatinous plate. The external coronal muscles of Atolla, which increases to 

 4 mm. broad by 2 mm. thick, and are composed of many layers of coronal muscular fibres, 

 lying the one above the other, form one of the strongest mesodermal masses of flesh (PI. 

 XXIX. figs. 4, 7, 8, one"). Among the Craspedotaa, Pedis furnishes an example of 

 strong mesodermal muscles in the velum and subumbrella (PI. V. fig. 7 ; PI. VI. figs. 

 12-14, on). 



§54. Nervous tissue (" tela nervea.") The two essential component parts of nervous 

 tissue, which are distinguished as nerve cells and nerve fibrillar in the higher animals, 

 are also already differentiated in the Medusae ; the two compose the central as well as 

 the peripheric part of the nervous system, even though their situation and connection 

 in the central nerve ring of the umbrella margin is different from that in the nervous 

 plexus of the subumbrella, and so forth. Both the nerve cells and the nerve fibrillse, 

 which proceed from and connect them, are, for the most part, products of the 

 ectoderm and have hitherto been considered to be exclusively such. Nerve cells and 

 nerve fibres are found in isolated places in some (and perhaps all) Medusae, which 

 originate from the endoderm, as on the very movable and sensitive oral parts (oral 

 filaments, oral lobes, oral arms, oral pouches). It is probable that on the gastral inner 

 surface of these oral organs the endoderm forms both muscular cells and nerve cells, 

 among the latter we may perhaps look for specific sense cells (gustatory cells ? olfactory 

 cells ?). These difficult conditions require much closer investigation. In any case, 

 both this localised and slightly extended ectodermal nervous tissue, and the more 



