THE ORGANIZATION OF THE PROTOZOA 



37 



either case the spicules grow by accretion that is to say, by deposi- 

 tion of fresh layers of inorganic substance upon that already laid 

 down and if such accretion takes place at one end of a rod-shaped 

 spicule, it may have the result that the opposite extremity of the 

 spicule is pushed outwards by the continued growth, with the result 

 that the oldest portion of the spicule projects freely far beyond the 

 limits of the body. 



As regards material, spicules are usually either calcareous or 

 siliceous in the first case generally carbonate of lime, in the second 



FIG. 18. Acanthocystis chcetophora, a Heliozoon with a skeleton of slender radiating 

 siliceous spicules, each forked at the distal end. In the interior of the body 

 are seen numerous symbiotic algae (dark) and non-contractile vacuoles (clear) ; 

 one vacuole of larger size is seen, probably the contractile vacuole. sp., sp., 

 Spicules ; pa., ps., pseudopodia. After Leidy, magnified 750 diameters. 



case amorphous silica In the family Acanthometridce among the 

 Radiolaria the spicules are formed of a substance which was thought 

 to be of organic nature, and was named " acanthin," but which 

 has been found to consist of strontium sulphate. 



As regards their form and relation to the body, the spicules in 

 the simplest cases are rod-shaped or needle-like elements disposed 

 radially or tangentially. A simple type of spicular skeleton is seen 

 in Acanthocystis (Fig. 18), in which elongated siliceous rods, fre- 



