STR UCTURAL DIFFERENTIA TIONS 



131 



such skeletons have theu- beginnings in spicules which conform in 

 shape and size with the nodal points in the alveolar walls of the cyto- 

 plasmic reticulum (Fig. 11, p. 33). Isolated spicules are character- 

 istic of several Heliozoa and Radiolaria where they form a loose 

 or felted covering in the outer protoplasm. Such spicules invari- 

 ably grow by accretion, that is, by the addition of new substance 

 to the outside of that already formed. If such added material 

 is formed in a limited region of the protoplasm, the result is a con- 

 tinued accretion of silica to the end of a spicule which is pushed 



Fig. G7. — Types of spicules in Heliozoa. ^4, Raplndiophrys pallida with curved 

 silicious spicules; B, Pinaciophora ruhiconda with tangential plates and forked spines; 

 C, Acanthocystis turfacea, with separated plates and forked spines; D, Pinaciophora 

 fluviatilis. (From Calkins after Penard.) 



farther out with each increment, thus giving rise to long bars and 

 spines which are radially arranged in forms like Acanthocystis 

 acvleata, etc. (Fig. 67). The silicious deposit, again, may be made 

 throughout a zone completely surrounding the center, resulting in 

 clathrate or latticed skeletons of varying grades of complexity 

 {Clathrulina elegans, NasseUario). 



Cellulose mantles are limited almost entirely to forms provided 

 with chlorophyll, and able to form starch by virtue of the energ^>' of 

 the sun's rays. Cellulose is a by-product of these cells and is secreted 

 either as a uniform covering for the entire organism, as in man\' 



