1666 



THE VOYAGE OF H.M.S. CHALLENGEE. 



thin wall of silex ; but they seem usually not to communicate with the alveoles of the 

 foot dii-ectly ; 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 (PI. 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 diff"erentiated 

 into two difi'erent and alternating groups, six descending and six ascending. The six 

 upper or ascending feet are arborescent, and each branch is armed a,t 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 spathillae, 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 

 spathillse ; and sometimes these terminal branches are very large (Pis. 118, 11 9). 



The central capsule of the Medusettida is usually subspherical or somewhat lenticular, 

 and hidden in the aboral or upper part of the sheU-cavity, whilst its oral or lower part 

 is fiUed up by the phseodium (PL 120, figs. 2, 9, 10, 11). Sometimes the phseodium 

 is very large and prominent at the aperture of the mouth (PL 118, figs. 2, 3; PL 

 119, fig. 1). The nucleus is usually eUij^soidal, 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 

 phseodium. 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. 



Si/nopsis of the Genera of Medusettida. 



Three equal feet, 



I. Subfamily 



Eupliysettida. 



Peristome of the shell with a corona of -j Four equal feet, 

 three or four feet. Apex of the shell 

 (usually) with an apical horn. 



II. Subfamily 



Gazellettida, 

 Peristome of the shell with sis to twelve ■ 



or more feet. Apex of the shell (usually) 



without apical horn. 



One large and three small feet, 



Six descending feet, .... 



Six descending and six ascending alternating 

 feet, ..... 



Xumerous (ten to twenty or more) descendiBg 

 feet, ..... 



699. Cortinetta. 



700. Medmetta. 



701. Euphysetta. 



702. Gazelletta. 



703. Gorgonetta. 



704. PolypeUa. 



