34 



IXVKRTEBRATE AXl.MALS. 



closely to the Foramiuifera ; and the resemblance between the two 

 groups is still further increased by the fact that the pseudopodia 

 often run into one another so as to form a network, and sometimes 

 show a circulation of granules along their edges. There is generally 

 a central capsule surrounded by a layer of sarcode exteriorly, and 

 the latter usvially contains coloured cells. A contractile vesicle is 

 usually wanting. Four principal groups of organisms have been 

 described as belonging to the Radiolaria, and we may briefly notice 

 an example of each of these. 



In the first family we have organisms like Acanthometra (fig. 

 13, a), in which the body is composed of sarcode, supported by a 

 framework of siliceous or flinty rods, which all meet in a common 

 centre. The spines or rods are all perforated by canals, and each con- 

 veys a pseudopodiuni, which is protruded from an aperture at its 



Fijj. 13, — a Acanthometra lanccolata ; h llaliomma Itexacanilmm, 

 una of the Polycystitm (after Muller). 



apex. Many pseudopodia, however, are given off from the surface 

 of the body directly, and are not enclosed in the spines. The Acan- 

 thometroi are all minute, and are found floating near the surface in 

 the open ocean, sometimes in great numbers. 



In the second family {Polycystiim, fig. 13, 6) we have a number of 

 beautiful little organisms allied to the Foramiuifera, but differing in 

 the fact that the body is enclosed in a glassy shell composed of flint. 

 The shell is perforated by nvimerous holes through which the pseudo- 

 ]3odia are emitted, and it is usually of extreme beauty, being sculp- 

 tured in various ways, and often adorned with spines. The sarcode 

 of the body is usually olive brown in colour, and often does not 

 quite fill the shell. 



The pseudopodia are filamentous, and exhibit a slow circulation of 

 granules along their borders, but they do not run into one another. 



