DERIVED ORGANIZATION— TAXONOMIC STRUCTURES L39 



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

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

 spines which are radially arranged in forms like Acanthocystis 

 aculeata, etc. (Fig. 75). 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, Nassellaria). 



While cellulose mantles and shells are more usually found in 



A D 



Fig. 75. — Types of spicules in Heliozoa. A, Raphidiophrys pallida with curved 

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

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



fluviatilis. (From Calkins after Penard.) 



chlorophyll-forming organisms, there are some types in which inter- 

 nal skeleton elements are composed of this or a closely related sub- 

 stance. In the parasitic Ophryoseolecidae skeletal structures are 

 present which are made up of a substance resembling cellulose to 

 which Dogiel gave the name Ophryoscolecin. 



(b) Motile Organoids.— The organoids by which Protozoa move 

 are to be considered as modifications of the cortex, although some 

 types, as shown in the preceding chapter, are derived in part 

 from internal kinetic elements (flagella and some pseudopodia). 

 Three main types are distinguishable flagella, pseudopodia and 

 cilia, each of which is sufficiently distinct from the others to furnish 

 a natural basis for classification of the Protozoa, a basis of classi- 



