The hollow articulate feet, or the large radial apophyses of the peristome, are the most remarkable organs of the Medusettida, and occur in the same form in no other family of Radiolaria. Their number is variable from three to twelve or more, and is here employed for the distinction of genera, as it is rather constant; the most frequent number is four (Medusetta) or six (Gazelletta). The feet are usually cylindrical, much longer than the shell, and more or less curved; more rarely they are straight or slightly compressed. They are always articulate, and divided by numerous transverse parallel septa into hollow compartments or alveoles; the transversely striated appearance of the feet is brought about by these septa. The alveoles have usually the form of short discoidal cylinders, and are broader than long (Pl. [120], figs. 5-13); rarely they are longer than broad (fig. 15). The septa between the alveoles are always perforated by a small opening and usually this opening is prolonged into a shorter or longer tubule, comparable to the siphuncle in the septa of the chambered shells of Cephalopoda; as in these latter, the succeeding siphons are arranged in a continuous series, which lies either in the axis of the foot or parallel to it (Pl. [120], figs. 8, 13, 15). The siphons are directed towards the distal end of the foot, so that, e.g., the siphon which arises from the septum between the ninth and tenth alveole, projects into the cavity of the latter. The communicating cavities of all the alveoles are filled up by jelly. In the dry shells, which are cleaned by hot mineral acids or by fire, and in which, therefore, the jelly is destroyed, the cavities become easily filled by air-bubbles (Pl. [120], figs. 8, 11, 15). Whilst the alveoles of each foot always form a long simple series, this series becomes doubled on the base of the larger feet, where they are broadened, and pass over into the compartments of the peristome (Pl. [122], figs. 10, 11, 14). The terminal alveole of each foot is closed.
The feet of the Medusettida are rarely quite simple and smooth (Pl. [120], figs. 1, 5, 7, 10, &c.). Usually they are armed with numerous spines or with elegant pencils of spathillæ. These secondary appendages or branches are also hollow, with a 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.
Synopsis of the Genera of Medusettida.
I. Subfamily Euphysettida. Peristome of the shell with a corona of three or four feet. Apex of the shell (usually) with an apical horn. |  | Three equal feet, | 699. Cortinetta. |
| Four equal feet, | 700. Medusetta. | ||
| One large and three small feet, | 701. Euphysetta. | ||
II. Subfamily Gazellettida. Peristome of the shell with six to twelve or more feet. Apex of the shell (usually) without apical horn. |  | Six descending feet, | 702. Gazelletta. |
| Six descending and six ascending alternating feet, | 703. Gorgonetta. | ||
| Numerous (ten to twenty or more) descending feet, | 704. Polypetta. |
I. Subfamily Euphysettida. Peristome of the shell with a corona of three or four feet. Apex of the shell (usually) with an apical horn. | ||||
| Three equal feet, | ||||
| 699. Cortinetta. | ||||
| Four equal feet, | ||||
| 700. Medusetta. | ||||
| One large and three small feet, | ||||
| 701. Euphysetta. | ||||
II. Subfamily Gazellettida. Peristome of the shell with six to twelve or more feet. Apex of the shell (usually) without apical horn. | ||||
| Six descending feet, | ||||
| 702. Gazelletta. | ||||
| Six descending and six ascending alternating feet, | ||||
| 703. Gorgonetta. | ||||
| Numerous (ten to twenty or more) descending feet, | ||||
| 704. Polypetta. | ||||
Genus 699. Cortinetta,[[316]] n. gen.
Definition.—Medusettida with three articulate feet on the peristome.
The genus Cortinetta comprises Medusettida which exhibit the minimum number of feet, three, and which therefore may be compared to the tripodal Nassellaria (Cortina, Cortiniscus, Tripodiscus, &c.). This similarity is the greater, as in the few observed species an apical horn is developed (as also in the following genus). The three feet are in the two species observed of equal size and similar form, and equidistant, so that they cannot be distinguished as an odd caudal and two paired lateral feet, as in the similar Nassellaria.
1. Cortinetta tripodiscus, n. sp. (Pl. [117], fig. 7).