RADIOLARIAXS. 



341 



XXVII. Another kind of Rhizopod, also inhabiting fresh water like the foregoing, are 

 the Sun-animalcules, or Heliozoa.* These, however, float free in the water, and have round bodies, 

 with delicate pseudopods radiating from all parts of the surface, producing a very gentle gliding 

 movement, and serving to capture prey. The sarcode in these animalcules has usually a yellowish 

 tint, and contains many clear globules or water-cavities, giving it a foamy appearance. Sometimes 

 it is "reen with either granular or diffused chlorophyll, derived from minute Algw taken in as food. 

 There are often red spots, also due to Algce. Besides one or more nuclei, there is a conspicuous 

 conti-actile or pulsating vesicle (sometimes more than one) ; and this in its action rises like a bubble 

 above the level of the surface, in some species, 

 and bursts so violently, as to shake the whole 

 animal and to make the discharge of its con- 

 tents evident in the surrounding water. 



Their body often presents the appearance 

 of a central granular mass, enclosed in a 

 capsule, but this is much more evident in some 

 of the allied marine ftadiolaria.-f The pseudo- 

 podia are thin threads of granular protoplasm, 

 tapering to extremely fine filaments, rarely 

 forked or branching at the ends. Though 

 straight, they are not rigid, but flexible and 

 contractile, drawing the food-atoms they touch 

 towards the body, where the particles are 

 enveloped by the sarcode and taken inside. In 

 some cases the pseudopods are said to be 

 strengthened by an internal axis of tougher 

 material. Sometimes the animalcule seems 

 to stand, as it were, on the ends of the 

 pseudopods touching the object beneath. 



Most of the Heliozoa are soft and naked ; 

 but others have an extremely rudimentary 

 skeleton of silicious % spicules in the outer 

 layer of sarcode ; and some have a more de- 

 veloped and delicate shell, of the same mineral substance, like lattice-work, forming an elegant 

 trellised sphere. 



This group corresponds to some extent with the marine JRadiofaria, and is termed " Fresh-water 

 Radiolarians " by some ; but they have a greater simplicity of constitution than most of the former. 



XXVIII. Acti)iophrys sol (Fig. 5), "the common Sun-animalcule, is one of the most familiar 

 and striking forms of microscopic life of still fresh water. ... It may be found in almost every 

 standing water-pool, pond, or lake, swimming among aquatic plants ; its favourite haunts being 

 duck-meat, hornwort, bladderwort, or the various filamentous Algae. It commonly appears as a 

 globular hyaline, foamy, or vesicular body, bristling with delicate rays, and suspended almost 

 stationary in the water." (Leidy.) It is about T V*h millimetre in size, and feeds on Rotifers, 

 Infusoria, unicellular Alga?, and Zoospores. Active animalcules touching its rays often seem to 

 be paralysed. Small prey glides down the pseudopods to their roots, where sarcode protrudes and 

 takes it in. Dr. "Wallich, in one of his memoirs, has figured an Actinophrys becoming itself a prey 

 to a large Amceba, which tore it piecemeal by means of its pseudopods, and engulfed a moiety 

 of it lump after lump. In his description of this circumstance, that careful naturalist remarks that, 

 however successful the stolid energy of the Actinophn/s usually may be, yet when an Amoeba comes 

 to the front the former avoids it ; but the latter with unusual activity endeavours to seize and to 

 envelope, or at least to tear out portions of the Actinophrys. 



* Greek, hdios, the sun ; zoon, an animal. f Latin, radiulus (diminutive of radius), a little staff or rod. 



+ Latin, silex, quartz or flint ; used for the kind of mineral comprising both these and other varieties of silica. 

 Greek, act is, a ray ; opkrys, the eyebrow. 



Fig. 5. ACMXOPHKYS SOL. 



The light spots are vesicles and water-cavities. The projecting globule is a con- 

 ti-actile vesicle about to burst. The dark spots are green zoo-pores of Alfffe, 

 taken in as food. The pwndopodB are here made much too thick. Magnified 

 500 diameters. (.4 fter Leidy.) 



