218 THE PROTOZOA 



III. Order Xenophyophora. With skeleton of foreign bodies 



and a peculiar internal structure. 



IV. Order Mycetozoa. Semi-terrestrial forms with repro- 



duction by resistant spores and formation of plas- 

 modia. 



B SUBCLASS ACTINOPODA (Calkins). Typically floating forms 

 with radiating, unbranched pseudopodia. 



V. Order Heliozoa. Principally fresh-water, without a 



" central capsule." 



VI. Order Radiolaria. Exclusively marine, with a central 

 capsule. 



I. AMfEB^BA. 



1. Reticulosa. In this suborder are comprised a number of 

 forms of doubtful affinities, sometimes ranked as a distinct order, 

 Proteomyxa. The only positive character which they have in 

 common is the possession of filose or retioulose pseudopodia, with 

 which is combined the absence of a shell and skeleton. Hence it 

 is not surprising that the position of many forms referred to this 

 suborder is extremely dubious, and some of them are referred to 

 distinct orders by many authorities. 



In general two types of organisms are referred to this suborder : 



(a) Large marine plasmodial forms ; an example is Poniomyxa 

 ftava, described by Topsent from the Mediterranean and British 

 Channel. Pontomyxa is a multinncleate plasmodium of yellow 

 colour. It sends out branching root-like pseudopodia, which may 

 spread out and form a network extending over two or three inches 

 in length. Nothing is known of its development or life-cycle. 



(6) Small forms with a single nucleus, marine or fresh-water, 

 which reproduce by process of multiple fission forming swarm- 

 spores. These forms have been subdivided into two families, 

 according to the type of swarm-spore found Zoosporidce, pro- 

 ducing flagellulae ; and Azoosporida, producing amoebulae. An 

 example of the Zoosporidce is furnished by the genus Pseudospora, 

 which preys upon algae, diatoms, Volvocineae, etc. The adult phase 

 is amcsboid, flagellate, or even Heliozoon-like. It feeds on the cell- 

 substance and chlorophyll of the prey, and multiplies by binary 

 fission. It can also break up by multiple fission into flagellate 

 swarm-spores, with or without previous encystment. Robertson 

 has observed syngamy between flagellulae thus formed, which are 

 therefore gametes ; in other cases the flagellulae are perhaps 

 agametes. As already pointed out above, the position of this form 

 amongst the Sarcodina is doubtful ; by many authorities it is 

 classified in the Mastigophora. 



An example of the Azoosporidce is furnished by Vampyrella, a 



