28 MANUAL OF ZOOLOGY SECT, i 



together as an order of Rhizopoda the order Heliozoa. 

 Of these other genera of Heliozoa Actinosphcerium (Fig. 7) 

 is somewhat more complex in structure than Actinophrys, the 

 protoplasm being divided into a central mass the endosarc 

 in which the vacuoles are small, and an outer layer the 

 ectosarc in which they are very large. Numerous nuclei 

 are present, and bodies containing chlorophyll the 

 characteristic green colouring-matter of plants. Some of 

 the Heliozoa, instead of being composed like Actinophrys 

 entirely of soft protoplasm, have supporting and protecting 

 hard parts. Such hard, or comparatively hard parts in any 

 animal, whatever form they may assume, whether that of 

 an enclosing shell or crust, or a system of internal bones or 

 other firm structures, are known under the general term of 

 skeleton. In those Heliozoa in which a skeleton occurs it is 

 sometimes a shell of agglutinated sand-grains, like the shell of 

 Difflugia or that of the arenaceous Foraminifera ; or it may 

 consist of loosely matted needle-like bodies composed of silica 

 (Fig. 8, /) ; or there may, as in Clathrulina (Fig. 8, j) be a 

 sphere of silica, perforated by numerous openings, enclosing 

 the protoplasm. Reproduction takes place, as in Amoeba, by 

 binary fission. But in certain genera the process of fission 

 under some circumstances remains incomplete, the two 

 protoplasmic bodies to which the fission gives rise 

 remaining connected together by a bridge or isthmus of 

 protoplasm, instead of becoming separated off in the shape 

 of two independent animals as in Amoeba. Further, these 

 two bodies may each in turn divide in the same incomplete 

 way, so that four Heliozoans are developed, all remaining 

 connected together ; and, by further repetitions of the same 

 process, a structure may be formed consisting of a large 

 number of units all connected together by living substance. 

 A structure of this kind, formed as a result of repeated 



