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



epithelium of the concave ventral umbrella (qw). The umbrella margin is the most 

 important part of the neurodermal system in all Medusae, both morphologically and 

 physiologically, as in it the most important animal organs — organs of sense, nerves and 

 muscles — attain their highest development. The central part of the nervous system and 

 the tentacles especially are always originally situated in the umbrella margin. The 

 umbrella margin is also of great importance for classification, as it is chiefly on it that 

 the variations of formation appear which lead to the distinction of genera and species. 

 In fact the distinction and nomenclature of the two principal divisions of the Class 

 Medusae, of the two sections Craspedotae and Acraspedas, are taken from the umbrella 

 margin, which presents important and striking diversities in the two sections. The 

 " velum " is characteristic of the former, the " lobe corona" of the latter. 



§ 67. Umbrella margin of the Craspedotse: velum ("diaphragma"). In all Craspedotae 

 or Hydromedusae a direct process of the free umbrella margin projects inwards from it ; 

 the marginal veil or "velum" (also termed "swimming membrane" or "diaphragma") 

 is wanting in all Acraspedae or Scyphomedusae. The velum forms a thin, membranous, 

 broader or narrower ring, which in a state of rest sometimes hangs loose vertically from 

 the umbrella margin, and is sometimes stiffly stretched horizontally and projecting inwards, 

 narrowing the entrance of the umbrella cavity more or less. In the Pectyllidas (Pis. III.— 

 VIII.) the velum is so broad that it Can probably close the entrance into the umbrella 

 cavity when fully extended. In most Narcomedusae it is very broad, whilst it is very narrow 

 in many Leptomedusae ; in Obelia it is rudimentary. We can always distinguish in the 

 velum a free distal margin and a basal proximal margin, inserted at the umbrella margin ; 

 likewise a ventral inner surface and a dorsal outer surface. The ventral or suburnbral 

 surface of the velum is covered with the ectoderm of the " suburnbrella," the dorsal or 

 exumbral surface with the ectoderm of the " exumbrella " ; below the latter there lies a 

 thin supporting plate, below the former a muscular plate, composed of circular fibres, 

 which is a direct process of the coronal muscular layer of the suburnbrella (comp. Pis. 

 IV. -VI., IX.-XIV.). 



§ 68. Collar lobes of the Narcomedusaa. Whilst in most Craspedotaa the velum is 

 stretched at equal breadth all round the umbrella margin, in the order of the Nar- 

 comedusae it undergoes peculiar transformations as the umbrella corona (or " collar "), and 

 is separated by deeper or shallower incisions of the margin into a number of separate 

 " collar lobes " (at least four, Cmiarcha, PL IX. ; usually eight or more, Pis. X.-XIV.). 

 These often closely resemble the true marginal lobes of the Acraspedae, and are usually con- 

 founded with them ; they differ entirely, however, in origin, structure, and signification. 

 The collar lobes of the Narcomedusaa originate in the tentacles, abandoning their original 

 insertion on the umbrella margin and migrating more or less upwards into the umbrella. 

 They then take with them a process from the urticating ring of the umbrella margin, in 

 the form of a radial (centripetal) urticating streak, which as the "umbrella clasp" 



