THE PEOTOZOA (continued) 



SECTION E. THE RADIOLARIA 1 



THE Eadiolaria are purely marine Gymnomyxa, specialised for 

 pelagic life. The body is usually spherical or conical, and emits 

 radiating thread-like pseudopodia. The cytoplasm is subdivided 

 by a perforated membranous " central capsule " into a central mass 

 and a voluminous mantle. The nucleus, which may be single or 

 multiple, is confined to the intracapsular region, which is also the 

 seat of reproductive changes, the extracapsular mantle being con- 

 cerned with flotation, feeding, stimulation, and excretion. A siliceous 

 skeleton is usually present, and may take the form of spicules, 

 shells, and tubes in a variety of delicate and exquisite constructions. 

 In one division (Acantharia) the skeleton consists, so far as is known, 

 of strontium sulphate. In most Radiolaria peculiar nucleated yellow 

 corpuscles are found in abundance. They are regarded either as 

 " symbiotic algae " or as Peridinians. Multiplication by fission is 

 known in a few cases ; more commonly reproduction by spore- 

 formation has been observed. 



DESCRIPTION or THALASSICOLLA. 



As an introduction to the description of the class the following 

 account of Thalassicolla has been drawn up. 



Thalassicolla is a spherical gelatinous Protozoon from 3-5 mm. 

 in diameter. In the warmer waters of the great oceans it occurs in 

 vast swarms that float passively at the surface but also descend 

 into deeper water during the reproductive phase. It ranges for some 

 forty degrees of latitude on either side of the equator, diminishing 

 in numbers towards these limits. It is abundant in the Faroe 

 Channel (Wolfenden, Fowler), and a stray specimen is now and 

 then recorded from our coasts (Delap [40]). 



Thalassicolla consists of two parts a central or medullary region 

 and a thick outer or cortical layer. The two are separated by the 

 central capsule. 



The intracapsular mass consists of a large centrally placed 

 nucleus embedded in cytoplasm, heavily laden with concretions, 



1 By F. W. Gamble, D.Sc., F.R.S., Manchester University. 

 94 



