28 MANUAL OF ZOOLOGY sect. 



movements. The pseudopodia owe their stiffness to the 

 presence of a rod of chitinoid material which lies in the axis 

 of each, and extends inward toward the middle of the pro- 

 toplasm. A large nucleus lies in the centre of the body. A 

 good many other genera are known which have pseudopodia 

 of the same general character as those of Actinophrys, and 

 these are accordingly grouped together as an order of 

 Rhizopoda — the order Heliozoa. Of these other genera of 

 Heliozoa, Actinosphcerium (Fig. 7) is somewhat more com- 

 plex 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. It frequently occurs in com- 

 pany with Actinophrys, among the leaves of Lemna and 

 other plants, and feeds on microscopic forms, also Rotifers 

 (Stokes). Some of the Heliozoa, instead of being composed 

 like Actinophrys entirely of soft protoplasm, have support- 

 ing and protecting hard parts. Such hard, or compara- 

 tively 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 aggluti- 

 nated sand-grains, like the shell of Difflugia, or of the arena- 

 ceous Foraminifera; or it may consist of loosely matted 

 needle-like bodies composed of silica (Fig. 8, 1) ; or there 

 may, as in Clathrulina, be a sphere of silica, perforated by 

 numerous openings', enclosing the protoplasm. Clathrulina 

 elegans (Fig. 8, 3) is common in many ponds, attached to 

 the rootlets of Lemna, or duck-weed (Stokes). 



Reproduction takes place, as in Amceba, by binary fission. 



