84 



PROTOZOA 



the last one opening by a pylonie (Fig. 25, B). These are termed 

 " thorax," " abdomen," and " post-abdomen " respectively. 



In the Phaeodaria the skeleton may be absent, spicular (of 

 loose or connected spicules) or latticed, continuous or bivalve. 

 It is composed of silica combined with organic matter, so that it 

 chars when heated, is more readily dissolved, and is not preserved 

 in fossilisation. The spicules or lattice-work are hollow, often 

 with a central filament running in the centre of the gelatinous 

 contents. The latticed structure of the shell of the Challengeridae 

 (Fig. 28) is so fine as to recall that of the Diatomaceae. In 

 the Phaeoconchida the shell is in two halves, parted along the 

 " frontal " plane of the three apertures of the capsule. 



Fiij. ^r.— Hclieiue of various possiljle skeletal forms deposited in tlie meshes of an 

 alveolar system, most of which are realised in tlie Radiolaria. (From Verworn, 

 after Dreyer.) 



The central capsule (rarely inconspicuous and dilticult, if not 

 impossil.)le to demonstrate) is of a substance which resembles 

 chitin, though its chemical reactions have not been fully studied 

 hitherto, and indeed vary from species to species. It is composed 

 of a single layer, except in Phaeodaria, where it is double. The 

 operculum in this group, i.e. the area around the aperture, is 

 composed of an outer layer, which is radially thickened, and a 

 thin inner layer ; the former is produced into tlie projecting tube 

 (" proboscis "). 



Reproduction in the Eadiolaria may be simple fission due to 

 the binary fission of the nucleus, the capsule, and the ectoplasm 

 in succession. If this last feature is omitted we have a colonial 

 organism, composed of the common ectoplasm containing numerous 

 central capsules; and the genera in which this occurs, all belonging 

 to the Peripylaea, were formerly separated (as Polycyttaria) from 



