40 



INVERTEBRATE ZOOLOGY 



Many important parasites, especially among the Amoebae and 

 Mycetozoa, occur in this group. The Radiolaria (Fig. 25) and 

 Foraminifera (Fig. 26) have been of great importance because 

 of the part played by their shells in the formation of sedimentary 

 rocks. 



Subclass ACTINOPODA 



The Heliozoa and Radiolaria are united to form the subclass 

 Actinopoda. The first of these are chiefly fresh-water forms, the 

 latter exclusively marine. As chiefly floating forms, the members 

 of this subclass are many of them spherical, with pseudopodia of 



Fig. 25. — Two characteristic forms of radiolarian shells. (After Haeckel). 



lobose type or with axopodia. A dense cortical layer of the cyto- 

 plasm so characteristic of Amoeba is lacking in the Actinopoda. 

 The cytoplasm is highly alveolar and in the fresh-water forms 

 (Heliozoa) bears one or more contractile vacuoles in the ecto- 

 plasm. A sharp separation of ectoplasm and endoplasm is 

 marked in the Radiolaria by a chitinous membrane called the 

 central capsule (Fig. 25 A). The cytoplasm within the central 

 capsule contains one or many nuclei, while the extracapsular 

 ectoplasm is devoid of nuclei and is differentiated into more or 

 less specialized zones. Minute openings in the capsular mem- 

 brane permit of communication between the ectoplasm and 

 endoplasm. 



Silicious skeletons of marvelously intricate design (Fig. 25 A) 

 or simple latticework cover the body in some members of this 

 subclass, though others may be naked or provided with varying 

 arrangements of spicules, spines, and plates. 



