EXPLANATION OF THE PLATES. 



(T) 



PLATES in., IV., v., and V^. 



Cyan'ea arctica, Per. and LeS. 



[All the figures of these plates were drawn from nature- by A. Sonrel.] 



Plate III. represents C'yanea arctica in one of its natural 

 attitudes, quietly floating near tlie surface of the water 

 with all its appendages hanging loosely down, most of 

 the tentacles being fully extended, and a few onlv 

 contracted. The attitude chosen makes it possible to 

 appreciate the position of the different parts, in their 

 natural relations, as seen in profile. One of the pillars 

 of the digestive cavity being in the centre of the figure, 

 two of the ovarian pouches are visible to the right 

 and left of it, behind the curtain formed by the four 

 bunches of tentacles of the same side. The crescent- 

 shaped line of insertion of the tentacles is well dis- 

 played by the two bunches on the right and left of 

 the pillar of the main cavity; it is foreshortened in 

 the two buiiclies occupying the margins of the figure. 

 Three eyes arc visible, one in the centre of the margin 

 of the disc and one on each side, a lobe without eye- 

 speck intervening between tliem. The festoon-like 

 ramifications of the chymiferous tubes in the lobes of 

 the margin of the disk are plainly visible, the disk beinn- 

 slightly contracted, in which case the margin is bent 

 downwards. The dark ridges in the centre of the 

 figure, terminating in sharp points, mark the outhnes 

 of the lower surface of the gelatinous disk, which is 

 of a rich reddish brown color, and forms the roof of 

 the main cavity of the body, exhibiting deep radiating 

 fiirrows arising from an even central flat disk. Be- 

 tween the margin of the disk and the pillars of the 

 digestive eavitj- appear the circular and the radiating 

 folds of the lower floor of the main cavity of tlic body, 

 and Mow the ovarian pouches and behind the ten- 

 tacles hang the folds of the prolongation of tlie oral 

 tentacles, which are more extensive in the geims C'ya- 

 nea than in any other Medusa. 



The specimen represented was an adult of ordinary size, 

 four times larger than the figure, which may give some 

 idea of the magnificence of such a Medusa when in 

 full activity, with all its tentacles stretching in everj- 

 direction. Specimens measuring three feet across the 

 disk are not rare in the Bay of Boston, in September, 

 and their tentacles may be seen trailing to a distance 

 of ten feet in every direction from the disk. In our 

 figure the lower ends of a large number of tentacles 

 are cut off. 'VATien perfectly undisturbed the tentacles 

 may be extended to an extraordiuan' lenjth. 



Plate FV^. represents our C'yanea from the lower surface, 

 with diSerent jfart.s removed, and reproduced by them- 

 selves. 



Fig. 1 may give a general idea of the relations of all 

 the parts visible from the lower side, some of them 

 being removed to allow the others to be seen in their 

 natural connection with the whole. Of the four lobes 

 extending from the four corners of the mouth, two 

 are entirely removed, and one (.«) is retained entire, its 

 two halves d' and d" being spread wide open to show the 

 medial furrow leading into the digestive cavity ; of the 

 fourth (.s') only one Iialf d is preserved with the medial 

 furrow, and the other half is cut off along the furrow. 

 One half (/ of the lobe s' and one half (/' of the lobe 

 6" are seen as they unite near the mouth, to show 

 how the four lobes are separated from one another, 

 and how their margins are folded all round. The four 

 ovarian jiouches alternate with these four lobes; but 

 only two are preserved in this figure, one of which 

 is almost entirely covered by the oral lobes of that 

 side, while the other is entirely uncovered, the two oral 

 lobes which hang to the right and left of it having 

 been removed. It is thus seen that the sexual pouches 

 hang down between the pillars of the corners of the 

 mouth, and lie in the centre of a ray terminating with 

 an eye o', each being flanked by two bunches of tenta- 

 cles lying in the direction of two lobes a" and a'", 

 in the centre of which there are no eyes. The cavities 

 of the sexual pouches open freely into the main cavity 

 of the body ; one of the cavities is laid open in the 

 direction of the eye o'", the walls of the pouches 

 being cut through near the pillars of the digestive 

 cavity. On the opposite side, the lower floor of the 

 main cavit_v is entirely removed in the direction of 

 the lobe a', while in the direction of the lobe a the 

 sexual pouch is alone cut off. The four oral lobes 

 alternating with the four sexual pouches are thus 

 seen to occupy the centre of eight rays, each of 

 which terminates with an eye, o o o o' o" o'", two 

 eyes being covered by the oral lobes s and s'. These 

 eight rays are the centres of the eight spheromercs 

 of which a C'yanea consists. Homologically speaking, 

 they are the eight ambulacral zones of the Cyanea. 

 With them alternate eight interambulacral zones, a a' 

 o" «'", the centres of the four others being covered 

 by a bunch of tentacles on the left side of the figure 

 and by the oral lobes at d" </' and '/. In the cen- 

 tres of these eight interambulacral zones there are eight 

 bunches of tentacles, three of which are covered by the 

 oral lobes preserveil in this figure, and one of which 



