RHIZOPODA. 41 
OrpDER IV. RapIOLARIA.—The third order of the RAzzepoda 
is that of the Radiolaria, essentially distinguished by the fact 
that the sarcode-body has the power of secreting a “siliceous” 
or flinty skeleton, either in the form of a shell, or of detached 
spicules or needles ; whilst the pseudopodia are long and thread- 
like, and stand out from the body like rays. In this last charac- 
ter the Radiolaria approach very closely to the Foraminifera ; 
and the resemblance between the two groups is still further in- 
creased by the fact that the pseudopodia often run into one an- 
other so as to form a network, and sometimes show a circulation 
of granules along their edges. Three groups of organisms have 
been described as belonging to the Radzolarza, and we may 
briefly notice an example of each of these. 
In the first family we have organisms like Acanthometra (fg. 
8, a), in which the body is composed of sarcode, supported by a 
Fig. 8.—a Acanthometra lanceolata; 6 Haliomma hexacanthum, 
one of the Polycystina (after Miiller). 
framework of siliceous or flinty rods, which all meet in a com- 
mon centre. The spines or rods are all perforated by canals, 
and each conveys a pseudopodium, which is protruded from an 
aperture at its apex. Many pseudopodia, however, are given off 
from the surface of the body directly, and are not enclosed in 
the spines. The Acanthometre are all minute, and are found 
floating near the surface in the open ocean, sometimes in great 
numbers. 
In the second family (Polycystina, fig. 8, 6) we have a number 
of beautiful little organisms closely allied to the Foraminifera, 
but differing in the fact that the body is enclosed in a glassy 
shell composed of flint. The shell is perforated by numerous 
