430 BIOLOGY OF THE PROTOZOA 



is made on the basis of pseudopodia types while minor groups are 

 based upon special structural or functional peculiarities. Thus one 

 great group is characterized by the possession of ray-like pseudo- 

 podia with axial filaments and is given here the taxonomic value of 

 Class I, the Actinopoda, and these show the nearest approach to the 

 Holomastigidae amongst the flagellates. A second group— Class II 

 —includes forms with myxopodia, filopodia and lobopodia and is 

 well termed, in recognition of Dujardin, the Rhizopoda. Possible 

 ancestral types for this group may be found in the Rhizomastigidae 

 amongst the Mastigophora. 



Class I. ACTINOPODA Calkins. 



These are usually homaxonic or spherical forms living for the 

 most part as suspended or floating organisms. Pseudopodia are 

 typically axopodia but lobose pseudopodia may also be formed, 

 mainly as food-taking organs. The protoplasm is highly alveolar, 

 becoming, in the ectoplasm particularly, vesicular or pseudo- 

 alveolar. A highly differentiated cortex is absent as well as the 

 denser cortical protoplasm which characterizes the Amebidae. In 

 fresh water forms (Heliozoa) one or more contractile vacuoles are 

 present in the vesicular ectoplasm. In the Radiolaria, ectoplasm 

 and endoplasm are sharply separated by a continuous chitinous 

 membrane— the central capsule— within which lie one or many 

 nuclei, while the extracapsular protoplasm is differentiated into 

 zones of more or less specialized ectoplasm. 



While several types are naked, the great majority of Actinopoda 

 are provided with spicules, plates, spines or skeletons often of 

 elaborate design and exquisite delicacy. Some forms are covered 

 with a gelatinous mantle in which foreign particles— diatom shells, 

 sand grains, etc.— are embedded. For the most part the spicules 

 and skeletons are composed of silica but in one large group of 

 Radiolaria, the Acantharia, they are horn-like and composed of 

 strontium sulphate. According to Dreyer spicules and skeletons 

 depend upon the vesicular configuration of the protoplasm and upon 

 the quantitv of mineral matter precipitated between the alveoli 

 (Fig. 12, p. 33). 



In Heliozoa a single vesicular nucleus is the rule, but there may 

 be from 200 to 300 in Actinosphaerium eichhornii and several nuclei 

 in Camptonema nutans. A multiple number is also characteristic of 

 the Radiolaria, or a single nucleus may become enormously enlarged. 



Nutrition is invariably holozoic, living organisms being captured 

 through the agency of lobose pseudopodia (Fig. 97, p. 180). Few 

 observations have been made, however, upon digestive processes or 

 final history of the food (see Chapter V). 



Reproduction occurs by division, either binary fission or unequal 



