Chap. III. ACTINAL SYSTEM OF CYANEA. 107 



cavity is so entirely surrounded by the innumerable folds of the curtain, that it 

 is entirely shut out of sight in the natural position of the animal ; even when 

 reversed, the mouth becomes visible only when the curtains are either removed or 

 stretched out horizontally, as in PL IV. Fi(j. 1. It is then seen that the corners 

 of the mouth jj resent an indentation corresponding to the middle of the heavier 

 fold of the arms, which forms the axis of the flowing curtains. These parts are 

 seen from the under-side in PI. IV. Fig. 1 ; they are seen in profile from the 

 inside in PI. V^ Fifj. 14, and from above in Fig. 16, in which the pillars of the 

 actinostome (i I), and the genital pouches (o s), are cut through at different 

 heights, in order the better to show the structure of these parts. On the right 

 side of the figure, these pillars are cut near their connection with the concentric 

 folds and shown to consist of two branches, separated from one another, as seen in 

 PL IV. Fig. 2, 1 1; while on the left side of the figure, they are cut immediately 

 above the transverse beams (5), so as to show that their two branches are here 

 close together, and pass in unbroken continuity into the rod-like main folds (s s) 

 of the flowing curtains. The gelatinous substance which gives strength to these 

 pillars extends also into the main fold of the curtains, and stretches even sideways 

 into the upper portion of that part of the curtains which is attached to the 

 transverse beams (PL V\ Fig. 17, 4). In this figure, o s is a portion of the 

 genital sac, 3 exhibits a transverse section of the horizontal beam, and 4 a section 

 of the gelatinous thickening of the lateral parts of the flowing curtains. The 

 horizontal beams, though stretching across from one pillar to the other, are slightly 

 arched outward, as Fig. 1, PL IV. shows. It is not difficult to understand how 

 the curtain-like portion of the actinostome is thrown into the innumerable folds it 

 presents, as seen in PL III. and PL IV. Fig. 1, and PL V". Figs. 14, 15 ; since 

 the main fold {s s), Avhich corresponds to the axis of each of these curtains, is, 

 properly speaking, homologous to the more solid portion of an arm of Aurelia, 

 while the flowing folds correspond to its thin margin ; only that in Cyanea these 

 margins are very long and thin, and grow broader and broader as they are further 

 removed from the medial line. At the same time, they are shorter near the junc- 

 tion of two arms {(^), and longest about mid length ; while the medial prolongation 

 of each arm becomes as thin and waving as its lateral folds. The natural con- 

 sequence of this arrangement is, that the main mass of the folds (PL Y\ Figs. 14 

 and 15, d), have nearly the same length, while those occupying the interval between 

 two arms {(i* d"^) are gradually shorter, up to the point where two bunches meet. 

 Compare also, PL IV. Fig. 1. 



Without being as active as the tentacles, the flowing curtains of the actinostome, 

 with their many folds, are in unceasing motion, rising, or falling, or spreading, in 

 parts, so that larger or smaller masses of these folds may be seen shortening or 



