THE ORGANIZATION OF THE PROTOZOA 39 



quently branched at their distal ends, are arranged like radii of the 

 spherical body, projecting freely for some distance from the surface. 

 In other cases the spicules may be disposed tangentially to the body, 

 as in the family Collidce amongst the Radiolaria, and in other forms 

 belonging to this order. From a simple type of skeleton composed 

 of separate spicules, more complicated types of skeletons are de- 

 rived by fusion of the spicules to form a connected framework. 

 The commonest type of this is a fusion of tangentially-disposed 

 spicules to form a lattice- work ; an example of this is seen in 

 Clathrulina (Fig. 19), in which a lattice-like skeleton is formed at 

 the surface of the body, standing off from it like a shell. Skeletons 

 of this type are especially characteristic of the Radiolaria, a group 

 In which the architecture of the skeleton may reach a very high 

 degree of complication and exhibits endless variety. The lattice- 

 like framework, made up of tangentially-arranged spicules united 

 together, may be further strengthened by radially-disposed beams. 

 As the animal grows, it may outgrow the framework first laid down, 

 and another lattice-work is formed concentric with the first, and 

 connected with it by radial beams ; later on a third and a fourth 

 such framework is formed, as the organism continues to grow in 

 size. Skeletons formed in this way may be " homaxon " that is to 

 say, built up on the axes of a sphere ; or ' monaxon," with one 

 principal axis ; or may follow various plans of symmetry, or may be 

 asymmetrical (p. 250, infra). 



Bibliography. For references, see p. 477. 



