852 MICROSCOPIC FOBMS OF ANIMAL LIFE 



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

 ramify more or less minutely, as in Arachnosphcera (fig. 647, 4). But 

 more frequently the shell opens out at one pole into a form more or 

 less bell-like, as in Podocyrtis (fig. 646, A, and fig. 644, a, o), Eliopalo- 

 canium (fig. 646, B), and Pterocanium (fig 645, B) ; or it may 

 be elongated into a somew r hat cylindrical form, one pole remaining 

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

 (fig. 644, d, (/, /). The transition between these 1 forms, again, proves 

 to be as gradational, when many specimens are compared, 1 as it is 

 among Foraminifera. 



Acanthometrina. In this group the animal is not inclosed within 

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

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

 centre (fig. 645, A). The soft sarcode-body is spherical in form, and 

 occupies the spaces left between the bases of these spines, which 

 are sometimes partly inclosed (as in the species represented) by 

 transverse projections. The ' capsule ' is pierced by the pseudo- 

 podia, whose convergence may be traced from without inwards, 



afterwards passing through it ; and it 

 is itself enveloped in a layer of less 

 tenacious protoplasm, resembling that 

 of which the pseudopodia are composed. 

 One species, the Acanthometra echin- 

 oides, which presents itself to the naked 

 eye as a crimson-red point, the dia- 

 meter of the central part of its body 

 being about T ^^ths of an inch, is very 

 common on some parts of the coast of 

 Norway, especially during the preva- 

 lence of westerly winds; and the 

 FIG. 651.Haliomma Humboldtii. Author has himself met with it abun- 

 dantly near Shetland, in the floating 



brown masses termed madre by the fishermen (who believe them 

 to furnish food to the herring), which consists mainly of this 

 Acanthometra mingled with Entomostraca. 



Phaeodaria. Among the most important of the Radiolaria 

 collected by the ' Challenger ' are the comparatively large (as much as 

 1 mm. in diameter) single-celled forms which are remarkable for the 

 constant presence of large dark brown granules, which are scattered 

 irregularly round the central capsule and cover the greater part of 

 its outer surface. The nucleus is large, the capsular membrane is 

 always double, and is pierced by one or more large openings ; the 

 whole cell is inclosed in a thick gelatinous covering, and there is 

 nearly always a well -developed extracapsular silicious skeleton, 

 according to the structure of which the group is subdivided. 2 



Collozoa. To this group belong those remarkable composite 

 forms which, exhibiting the characteristic rarliolarian type in their 



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. xiii. 1865, p. 75. 



2 On reproduction in this group, cf. A. Borgert, Zool. Anzeig. xix. (189(5), p. 307. 



