848 



MICROSCOPIC FORMS OF ANIMAL LIFE 



little flexibility : in some species (but not ordinarily) they branch 

 and anastomose, while in others they are inclosed in hollow 

 rods that form part of the silicious skeleton, and issue forth from 

 the extremities of these. A flow of granules takes place among 

 them ; and the mode in which they obtain food-particles (consisting 

 of diatoms and other minute alga?, marine infusoria, etc.), and draw 

 them into the sarcode-bodies of the radiolarians, appears to corre- 

 spond entirely with their action in Actinophrys and other Heliozoa. 

 The yellow cells, or Zooxanthellce, as K. Brandt has proposed to 

 call them, so often seen in these cells, are not confined to Radiolaria, 



A B 



\ 



FIG. 645. Polycystina : A, HaUomnni Injuh-ir ; B, Pterocaniiim, with animal. 



for they are found also in Actinia? and various other invertebrates; 

 nor are they always present in examples studied ; they are now com- 

 pletely recognised ' as alga' which form a 'symbiotic ' relation with 

 their host, the animal profiting by the removal of its waste products 

 by its messmate, by the oxygen which its guest evolves in sunlight, 

 and by the food-material it provides after death, while the plant 

 feeds on the waste of the animal. 



In most Radiolariit skeletal structures are developed in the 

 .samxle-body, either inside or outside the capsule. or m both positions; 

 sometimes in the form of investing networks having more or less of 

 a spheroidal form (fig. 647, 1, -2). or of /-ti>/ it/tin;/ spines, 3, or of 

 comliiiiat ions of these, 4, 5. But in many cases the .skeleton consists 

 only of a lew scattered spicules ; and this Ls especially the case in 

 certain large composite loi-ms or ' colonies ' (fig. <>.VJ). which may 



K. i;i,mclt, Verhandl. Physiol. Gesellsch. Berlin, 1881-82, p. 22 ; 

 Milth. /.<!<,!. Stat. Neapel, iv. [>. 191; P. Geddes, Nature, \\\. \>. ::ii:'.. 



