PROTOZOA : RADIOLARIA. 



79 



The following are the more important groups of the Radio- 

 laria : 



I. FAMILY ACANTHOMETRINA. The Acanthometrce (fig, 18, a) 

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



Fig. 18 a Acanthometra lanceolata; b Haliomma^ hexacanthum, one of the 

 Polycystina, showing the radiating pseudopodia. (After Mailer. ) 



open ocean, sometimes in great numbers. They consist of 

 sarcode-bodies, which are supported by a framework of radiat- 

 ing siliceous, or horny spines, the extremities of which usually 

 project considerably beyond the body. The substance of the 

 body admits of division into an outer membranous layer, or 

 " ectosarc," and an internal granular layer, or " endosarc." 

 The siliceous spines are hollow, being grooved at the base by a 

 gutter, which is continued further up the spine by a canal ter- 

 minating at the apex of the spine by a distinct aperture. The 

 spines, in consequence of this structure, are able to serve for 

 the transmission of the pseudopodia, which gain the exterior 

 by running through the canals and escaping at their apices. 

 Many of the pseudopodia, however, do not occupy the canals 

 of the spines. 



II. FAM. POLYCYSTINA. The members of this family are 

 closely related to the Foraminifera, differing from them chiefly 

 in the fact that their shells are composed of flint instead of 

 carbonate of lime, as in most of the latter. They possess a 

 body of sarcode, which is enclosed in a foraminated siliceous 

 shell, which is often furnished with spine-like processes, and is 

 usually of great beauty (figs. 17 and 18, b). The sarcodic 

 substance of the body is olive-brown in colour, with yellow 

 globules, and often does not entirely fill the shell. The pseudo- 

 podia are emitted through the foramina in the test, and are 

 long, ray-like filaments, which display a slow movement of 

 granules along their borders. 



