I 3 6 THE RADIOLARIA 



sustentative and propulsive forces. When these blades are wanting 

 and the spines are merely flattened, they are set in each of the 

 three circles, so as to turn the flat edge somewhat differently to the 

 water, the equatorial ones lying flat on the water, the tropical ones 

 turned half over, and the polar spines set on edge. By this means 

 the amount of resistance to the water in every direction is increased. 

 The exceptionally wide distribution of the Acanthometrida is some 

 confirmation of these deductions. 



Fission Reproduction. The phenomena of multiplication and of 

 reproduction are still imperfectly known. Binary or multiple 

 fission occurs in some Spumellaria, Acanthometrida, and Phaeodaria. 

 Gemmation is a rarer mode. It produces the extracapsular bodies 

 of the Sphaerozoidae, and is found in one species of the Acantharia 

 and of the Phaeodaria respectively. The development of zoospores 

 is a general phenomenon, but has been followed in detail only in a 

 few cases. Plastogamy is unknown. 



The mode of increase by fission is probably restricted to those 

 Radiolaria which have no spicules or a lax and osculate skeleton. 

 Binary fission occurs in the Thalassicollidae, some Acanthometridae, 

 and in* two families of Phaeodaria. Division both of the Sphaero- 

 zoid colony and of its component individuals takes place at intervals. 

 Multiple fission occurs in the Thalassophysidae. The process is 

 usually initiated by changes in the endoplasm and nucleus, and a 

 long interval may follow before any corresponding alterations occur 

 in the ectoplasm (Phaeodaria). 



In the Acanthometrida (Acanthoniidae) binary, quatenary f 

 and multiple fission are said to occur (Popowsky). The former 

 process is illustrated in Fig. 20. The skeletal rods separate at 

 their central ends into two bundles, the nuclei segregate into two 

 groups, the central capsule divides, and ultimately fission takes 

 place. The fission - products are, however, asymmetrical, and 

 Fig. 20 shows how the new radii are developed and how the rods 

 are swung into position, probably by contraction of the myonemes 

 inserted into them, until the whole arrangement is brought into 

 conformity with Miiller's law. 



Fission in the Phaeodaria is carried out in several ways. 

 Aulacantha scolymantha is the best -known example of the direct 

 process. In this Radiolarian the large single nucleus divides either 

 by mitosis or amitotically ; the endoplasm segregates round the 

 daughter nuclei ; the central capsule, after disappearing for a time, 

 re-forms about the two masses. Lastly, the phaeodial complex, the 

 calymma and spicular skeleton are subdivided each into two groups, 

 and the whole organism divides into two. In the Phaeodaria, 

 which possess a shell, one or more modifications of the process are 

 found. The helmet-shaped Challengeridae, for example, undergo 

 fission within the shell. One half of the organism now escapes 



