Chap. III. ABACTINAL SYSTEM OF CYANEA. U3 



gradually in thickness in the part nearest the long junctions, so that here the disk 

 remains comparatively thicker, near the margins, than is the case at the peripheric 

 end of the short junctions, where it suddenly loses its thickness in rounded outlines, 

 passing obliquely towards the peripheiy in the direction of the crooked lines. The 

 natural consequence of this disposition is that the part of the disk which embraces 

 the deep indentations is comparatively thin, and remains so to a greater distance 

 from the margin ; while the part embracing the lesser indentations is comparatively 

 thicker. Besides this, there is a deep furrow along the short and the long junctions, 

 and a prominent keel along the crooked lines. The colored pigment, however, covers 

 only the bulging, rounded part of this surface, but does not extend over the crooked 

 lines, nor over the short and the long lines of junction of the segments of the disk, 

 so that these lines naturally appear more transparent than the spaces which they 

 circumscribe. 



From this it will easily be understood why the disk, seen from above, presents, 

 as the optical effect of its structure, the various lines already described, and how 

 important it must be for those engaged in drawing Acalephs to understand this 

 accurately, in order correctly to represent what they see. The figures of a great 

 many Discophora?, published bj' different authors, and especially those in the voy- 

 ages of the Uranie and of the Coquille, however beautiful in their appearance, 

 represent these lines as surface features of the Medusoe. Mr. Sonrel, who has 

 drawn the plates quoted above, has succeeded admirably in reproducing the trans- 

 parency of the gelatinous disk, in such a way as to make it apparent that all these 

 outlines are only the optical eiFect of structures seated on the lower surface of the 

 disk or in its thickness, and not upon its upper surface. A comparison of Plate 

 V. with Plate III. confirms plainly this impression, as in the latter figure the furrows 

 following the long and the short junctions appear like keels in the direction of 

 the deeper and lesser eraarginations, and the inequalities which mark the tessellate 

 appearance of the lower surface of the central circular area are visible as slight 

 prominences within these ridges. 



The most marked depression observed upon the upper surface of the disk lies 

 in the jDrolongation of the long junction, near the margin; — it is scooped out, so 

 as to render that portion of the long junction thinnest which extends immediately 

 above the ocular apparatus. The spaces of each gelatinous mass, contained between 

 the long junction and the adjoining crooked lines, are so bulging towards the circum- 

 ference, that the small lobes are thickest in the middle and thinnest along; their edtre. 

 PI. V^ Fig. 1. This is particularly well seen, when the lobes are bent downward, 

 as in PI. IV. Fig. 6 and PI. V. Fig. 2. The spaces between the short junctions 

 and the adjoining crooked lines are also bulging, so that the large lobes are 

 likewise thicker in the middle than on the margin, and this, again, is best seen 



