THE RADIOLARIA 



109 



easily characterised by the brown, greenish-brown, or black accumu- 

 lation of food material, debris, and resistant " phaeodellae " that lie 

 in the oral half of its ectoplasm ; and they are also signalised by 

 the mode of distribution of the capsular pores. In the majority of 

 genera the endoplasm communicates with the ectoplasm only by 

 a teat- like operculum and a pair of small lateral conical pores 

 (the so-called astropyle and parapyles). In a few cases two 

 astropyles occur, and in at least one genus (Atlanticella) only a 

 single pore-plate is present. The skeleton varies greatly in structure 



Fia. 7. 



Aulactinium actinastnim, H. ; a member of the Phaeodaria. (After Haeckel, slightly 

 modified.) A, astropyle ; C, calymma ; AT, double nucleus lying in the endoplasm ; P, 

 parapyle ; Ph, phaeodium. 



and configuration. It is usually of a tubular nature, and the hollow 

 cylinders are often subdivided by septa. The basis of these 

 tubes, however, is formed by minute aciculate spicules which are 

 surrounded by a gelatinous sheath, and between this sheath and 

 the surrounding ectoplasmic matrix is a thin membrane, which first 

 becomes silicified. This is followed by deposition of silica in the 

 gelatinous sheath, and in this way complex spicules, often with 

 candelabra-like appendages, are developed. A single or double per- 

 forated shell may be present, the surface of which has a peculiar 

 porcellanous appearance and " diatomaceous " structure. In the 



