REPORT ON THE RADIOLARTA. 1665 



scattered over the surface (PI. 116, figs. 1, 2). In some genera a long vertical or 

 oblique spine is developed from the apex of the shell, and is comparable to the " apical 

 horn of the NASSELLARIA (PI. 118, figs. 2, 3; PL 120, figs. 1-4). 



The mouth of the shell, or the large opening on the lower pole of its vertical main 

 axis, is usually widely open, circular, or polygonal, more rarely constricted. In many 

 species the peristome is broadened and its margin developed in the form of a horizontal 

 diaphragm, which constricts the entrance into the shell-cavity, and is comparable to 

 the velum of the Hydromedusse (PL 120, figs. 1-4). Sometimes the margin of the 

 velum is fringed and bears numerous small dependent filaments of silica, comparable to 

 the marginal tentacles of many Medusae (PL 118, figs. 1, 3 ; PL 120, fig. 16). Rarely 

 the mouth is strongly constricted and prolonged into a narrow, long, cylindrical tube, 

 comparable to a proboscis ; its opening is usually fringed (PL 116, figs. 1, 2). 



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. 513) ; 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 spathillse. These secondary appendages or branches are also hollow, with a 



(ZOOL. CHALL. EXP. PAKT XL. 1886.) Rr 209 



