FORAMINIFERA AJ*D RADIOLARIA. 



113 



of no more account in these low forms of Animal life, than it is in the 

 discoidal Diatoms ( 290). Other discoidal forms, showing a like com- 

 bination of radial and circumferential parts are represented in Figs. 347 

 and 348, and also in Fig. 345, e, m. 



503. Entosphcerida. In this group the siliceous shell is spheroidal, 

 and is formed within the capsule; and it is not traversed by radii, al- 

 though prolongations of the shell often extend themselves radially out- 

 wards, as in Cladococcus (Plate xix., fig. 5). Sometimes the central 

 sphere is inclosed in two, three, or even more concentric spheres con- 

 nected by radii, as in the beautiful Actinomma (Plate xix., fig. 2); 

 reminding us of the wonderful concentric spheres carved in ivory by the 

 Chinese. One of the most common examples of this group is the Hali- 

 omma Humboldtii (Fig. 349), in which the shell is double. 



504. Polycystina. This name, which originally included the pre- 

 ceding group, is now restricted to those which have the shell formed 

 outside the capsule. This shell may, as in the preceding, be a simple, 

 sphere composed of an open siliceous network, as in Etlimo splicer a, (Plate 

 xix., fig. 1); or it may consist of two or three concentric spheres con- 

 nected by radii; or, again, it may put forth radial outgrowths, which 



FIG. 34T. 



FIG. 348. 



Perichlamydium prcetextum. 



Stylodyctya gracHis. 



sometimes extend themselves to several times the diameter of the shell, 

 and ramify more or less minutely, as in Arachno splicer a (Plate xix., fig. 

 4). But more frequently the shell opens-out at one pole into a form 

 more or less bell-like, as in Podocyrtis (Plate xvin., fig. 1, and Fig. 345, 

 , o), Rhopalocanium (Plate xvm., fig. 2), and Pterocanium (Plate xvni., 

 fig. 4); or it may be elongated into a somewhat cylindrical form, one 

 pole remaining closed, while the other is more or less contracted, as in 

 Eucyrtidium (Fig. 345, d, g, i). The transition between these forms 

 again, proves to be as gradational, when many specimens are compared, 1 

 as it is among Foraminifera ( 488). 



505. Acantlwmetrina. In this group the animal is not inclosed with- 

 in a shell, but is furnished with a very regular skeleton composed of 

 elongated spines, which radiate in all directions from a common centre 

 (Plate xix., fig. 3). The soft sarcode-body is spherical in form, and 

 occupies the spaces left between the bases of these spines, which are some- 

 times partly inclosed (as in the species represented) by transverse projec- 



1 The general Plan of structure of the Polycystina, and the signification of 

 their immense variety of forms, were ably discussed by Dr. Wallich, in the 

 "Trans, of the Microsc. Soc., ' N.S., Vol. xii. (1865), p. 75. 

 8 



