Page:Scientific results HMS Challenger vol 18 part 2.djvu/790

1666 thin wall of silex; but they seem usually not to communicate with the alveoles of the foot directly; the cavities of the foot and its appendages are separated by a thin, solid septum. In Medusetta the convex outer edge of each curved foot is studded with a series of simple alveolate branches (Pl. 120, figs. 2-4). In Gazelletta, the feet are usually armed with verticillate or alternate spines, which are either simple or branched (Pl. 120, figs. 11-15); sometimes each branch is elegantly arborescent (Pl. 118, fig. 1). Sometimes the distal end of each branch is armed with a spathilla or a coronet of recurved teeth. The branches attain their highest development in the admirable Gorgonetta (Pl. 119). The twelve feet of this most interesting genus are differentiated into two different and alternating groups, six descending and six ascending. The six upper or ascending feet are arborescent, and each branch is armed at the distal end with a spathilla (figs. 1, 2). The six lower or descending feet are covered with most elegant arborescent pencils or anchor-bearing trees; each terminal branch of a tree is armed with two spathillæ, a smaller terminal and a larger below it (figs. 3, 4). The distal ends of the feet are rarely simple, usually they are forked or branched, or armed with peculiar spathillæ; and sometimes these terminal branches are very large (Pls. 118, 119).

The central capsule of the Medusettida is usually subspherical or somewhat lenticular, and hidden in the aboral or upper part of the shell-cavity, whilst its oral or lower part is filled up by the phæodium (Pl. 120, figs. 2, 9, 10, 11). Sometimes the phæodium is very large and prominent at the aperture of the mouth (Pl. 118, figs. 2, 3; Pl. 119, fig. 1). The nucleus is usually ellipsoidal, half as large as the central capsule, and includes numerous nucleoli. The membrane of the central capsule seems in all Medusettida to possess only one opening, the astropyle or main-opening with a radiate operculum and proboscis, placed at the lower or oral pole (Pl. 120, fig. 2). I have never been able to observe any secondary opening or parapyle. The free space between the shell wall and the capsule is filled up by the calymma, which also includes the phæodium. The jelly of the calymma is probably in direct continuity with the jelly which fills up the alveoles of the shell-wall and of the articulate feet.