256 HANDBOOK OF PROTOZOOLOGY 



possess a great contractile power, supposedly with hydrostatic 

 function. 



The skeletal structure of the Radiolaria varies considerably 

 from simple to complex and has taxonomic value. The chemical 

 nature of the skeleton is used in distinguishing the major sub- 

 divisions of the group. In the Actipylea it seems to be made 

 up of strontium sulphate, while in the three other groups, 

 Peripylea, Monopylea and Tripylea, it consists fundamentally 

 of silicious substances. The skeleton of the Actipylea is sharply 

 marked from that of others in form and structure. The ma- 

 jority of this group possess twenty rods arranged in a diverse 

 way, and radiating from the center. The rod-shaped skeletons 

 emerge from the body in most cases along five circles, which are 

 comparable to the equatorial, the two tropical and the two 

 circumpolar circles of the globe. This is known as Miiller's law, 

 since J. Miiller first noticed the arrangement in 1858. 



The life history of the Radiolaria is very incompletely 

 known (Fig. 102). Binary or multiple fission or budding has 

 been observed in some Peripylea, Actipylea, and Tripylea. 

 Multiple division is known to occur in Thalassophysidae, in 

 which it is the only known means of reproduction. The central 

 capsule becomes very irregular in its outline and the nucleus 

 breaks up into numerous chromatin globules. Finally the cap- 

 sule and the intracapsular cytoplasm become transformed into 

 numerous small bodies, each containing several nuclei. Fur- 

 ther changes are unknown. 



Formation of swarmers is known in some forms. In Thal- 

 lassicolla, the central capsule becomes separated from the re- 

 maining part of the body and the nucleus divides into a number 

 of small nuclei, around each of which condenses a small ovoidal 

 mass of cytoplasm. These small bodies soon develop flagella. 

 In the meantime the capsule descends to a depth of several 

 hundred meters, where its wall bursts and the flagellate swar- 

 mers are liberated. Two kinds of swarmers, isoswarmers and 

 anisoswarmers, are recognized. The former often contain a 

 crystal and a few fat globules. Of the latter, the macroswarmers 

 possess a nucleus and refringent spherules in the cytoplasm, 

 while the microswarmers contain a granular nucleus. Some 

 forms possess two flagella, one of which is coiled around the 



