PROTOZOA: RADIOLARIA. 67 



CHAPTER IV. 

 RADIOLARIA. 



ORDER IV. RADIOLARIA. The order Radiolaria was 

 founded by Miiller to include the Polycystina, the Acanthomet- 

 rina, and the Thalassicollida, to which Dr Carpenter adds 

 Actinophrys and its allies, chiefly on account of the form of the 

 pseudopodia. Here, however, the term will be employed to 

 designate the first three of these, and Actinophrys will be 

 placed amongst the Amoebea, to which its alliance appears to be 

 more decided. Most of the Radiolaria are marine, but some 

 few forms of Thalassicollida have been described as occurring 

 in fresh water. 



The order Radiolaria may be defined as comprising those 

 Rhizopods which possess a siliceous test or siliceous spiciiles, and 

 are provided with pseudopodia which stand out like radiating fila- 

 ments, and occasionally run into one another. 



i. FAMILY ACANTHOMETRINA. The Acanthometra (fig 10, a) 

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

 open ocean, sometimes in great numbers. They consist of 

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

 ing siliceous spines, the extremities of which usually project 



Fig. 10. a. Acanthometra lanceolata ; b Haliomma hexacantkum, one of the 

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



considerably beyond the body. The substance of the body 

 admits of division into an outer membranous layer, or " ecto- 

 sarc," 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 



