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



belong to the central umbrella discus. The peripheric umbrella corona, on the other 

 hand, is characterised by the broad coronal muscle of the subumbrella, and, above all, by 

 the umbrella margin, on which the central nerve system, and also the sense organs and 

 tentacles lie, besides these the velum in the Craspedotse and the lobe corona in the 

 Acraspedse. 



§ 36. Topographical antitheses. It is indispensable for the clear, and detaded ana- 

 tomical description of the Medusae, to avoid the usual, but indefinite and dubious terms, 

 " inner and outer, upper and lower " parts, &c, and instead of those to use definite topo- 

 graphical terms. For this purpose we represent the Medusa in its usual natural position 

 with the vertical principal axis, the convex umbrella surface turning upwards, the concave 

 umbrella surface turned downwards ; the umbrella margin forms the boundary between 

 them, in the more bmited sense, the free velum margin in the Craspedotas, the margin 

 of lobes in the Acraspedaa. All the parts lying above this free margin (or the upper con- 

 vex surface) we term dorsal or exumbral, all lying below it (on the lower, concave surface), 

 ventral or subumbral ; the marginal organs lie between the two on the umbrella margin. 

 With regard to the two poles of the vertical principal axis, we term all the central parts 

 which are turned towards its upper pole or apical pole " aboral," and those which are 

 directed towards the lower pole or oral pole " oral." Finally, as regards the two poles of 

 the radii or transverse axes, we name all parts of it which approach the central principal 

 axis " proximal parts," whilst those which turn towards the peripheric margin are " distal " 

 parts. 



§ 38. Organic systems. All the different organs which are developed in the Medusae 

 may be divided into two large organic systems, the neurodermal and the gastrovascular 

 system. The neurodermal system includes preferably the animal organs and apparatus ; 

 the umbrella with its exumbral umbrella-covering and subumbral muscular plate, the 

 umbrella margin, with the most important organs of animal life, the central nervous 

 system, the tentacles and the organs of sense. The gastrovascular system, on the other 

 hand, consists principally of the vegetative organs of nutrition and reproduction, of the 

 central principal intestine (with stomach and mouth), and of the peripheric coronal 

 intestine (with pouches and canals), and also of the genitalia or reproductive glands, 

 which are invariably developed in the subumbral wall of the gastrovascular system (some- 

 times from the ectoderm, sometimes from the endoderm). Taken altogether, the neuro- 

 dermal system finds its most important site of formation in the peripheric umbrella corona, 

 the gastrovascular system in the central umbrella disk ; the former is preferably 

 ectoblastic, the latter endoblastic. 



