ORDERS: AMCEBIDA AND HELIOZOA 



165 



shell, and grows into a many-chambered individual of another type 

 (megalospheric form), while the cytoplasm within the shell again gives 

 rise, by multiple segmentation, to daughter individuals (Fig. 74, A to C). 

 In this case, each daughter form which escapes from the shell is provided 

 with two flagella, by means of which it swims about till it meets another 

 similar form which has been produced by another individual. Conjuga- 

 tion takes place, and the zygote, losing the flagella, becomes an amoeba, 

 which forms a small shell and grows into a many-chambered individual 

 of the first type (microspheric form). In the great majority of the 

 Rhizopoda, however, no sexual process has been observed. 



The class Rhizopoda may be sub-divided into the five orders: 

 AMCEBIDA, HELIOZOA, RADIOLARIA, FOEAMINIFERA, AND MYCETOZOA. 



1. Order: AMCEBIDA Calkins, 1902. 

 The body consists of cytoplasm unprotected by any shell or skeletal 

 structure, while movement is effected by the formation of pseudopodia 

 from any part of the body sur- 

 face. There is usually a single 

 nucleus, but some forms have 

 two and others many nuclei. 

 The cytoplasm is generally dif- 

 ferentiated into a softer and 

 vacuolated inner portion, the 

 endoplasm, in which the nucleus 

 and food materials lie, and an 

 outer, more hyaline, and clearer 

 layer, the ectoplasm. This order 

 includes the organisms which 

 are generally known as amoebae, 

 and to it belong the various 

 parasitic forms which occur in 

 the intestinal canal of man and 

 animals. 



2. Order: HELIOZOA Haeckel, 

 1866. 



Fig. lo.^Actifiophrys sol ( x ca. 600). (From 



MiNCHIN, 1912, AFTER GRENACHER.) 



N., Nucleus from which radiate the axial fibres of 

 the pseudopodia; ps., pseudopodia; ax., axial 

 ; C.V., contractile vacuole;/.?;., food vacuole. 



The forms included in this 

 order have a characteristic radial 



appearance, the result of fine spiky pseudopodia (axopodia), which are 

 stiffened and rendered permanent by axial fibres. The latter may radiate 

 from a granule, probably centrosomic in nature, situated at the centre of 

 the organism, while the nucleus lies to one side of this (Fig. 51). The 



