318 BIOLOGY OF THE PROTOZOA 



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

 ancestral t\'pes for this group may be found in the Rhizomastigidse 

 and the Order Chrysomonadida, 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 Amoebidae. 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. For the most part these 

 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 quantity of mineral matter precipitated between the alveoli 

 (Fig. 11, p. 33). 



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

 be from 200 to 300 in Actinosphceriwn eichhornii and several nuclei 

 in Camyionema 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. 88, p. 179). Few 

 observations have been made, however, upon digestive processes or 

 final history of the food (see Chapter IV). 



Reproduction occurs by division either binary fission or unequal 

 division in the form of budding. Multiple division is frequent in 

 Radiolaria where the endoplasm gives rise to a multiple mmiber of 

 flagellated swarmers which may be of similar or dissimilar size 

 (isospores and anisospores). In some cases both kinds are formed 

 within the same central ca])sule. Whether these are gametes is a 

 matter, which, while probable, has not been satisfactorily proved. 



The Actinopoda are divided into two fairly well-defined sub-classes 

 —the Heliozoa of Haeckel, and the Radiolaria of Joh. Miiller. 



