78 THE INVERTEBRATA 



Order HELIOZOA 



Sarcodina, generally of floating habit and freshwater habitat ; without 

 shell or central capsule; sometimes with siliceous skeleton; with 

 spherical bodies ; typical axopodia ; and usually a highly vacuolated 

 outer layer of protoplasm. 



The locomotion of members of this group, in the ordinary phase, 

 is effected as rolling, due to contraction of successive pseudo- 

 podia in contact with the ground so that the body is pulled over. The 

 pseudopodia usually show streaming of granules. When they bend, 

 which they do to press towards the body prey which has adhered to 

 them, their axial filaments are temporarily absorbed at the bend. 



Contractile vacuoles are present. 



Asexual reproduction is usually by binary fission (or plasmotomy in 

 multinucleate forms), sometimes by budding. Sexual processes have 

 only been thoroughly investigated in Actinophrys and Actinosphaerium, 

 where they take the form of autogamy (see below). 



Dimorpha (Fig. 70), one of the Helioflagellata, a small group of 

 organisms which is usually appended to the Heliozoa, bears somewhat 

 the same relation to that order that Naegleria bears to the Amoebina. 

 It has a biflagellate and a heliozoan phase, and can pass from one to 

 the other. In the latter it retains the flagella, whose filaments share 

 a common basal granule with those of the axopodia, and has no 

 vacuolated layer or protecting case. In fresh waters. 



Actinophrys (Figs. 71 , 72). Unprotected ; with one nucleus, against 

 which the central filaments of the axopodia end ; no skeleton. Auto- 

 gamy (or more correctly paedogamy^) takes place as follows: the 

 pseudopodia are withdrawn and a jelly cyst formed. Binary fission 

 now takes place, so that two individuals lie side by side in the cyst. 

 Each divides mitotically twice, throwing out as a polar body one 

 product of each division. The first of these two divisions is a reduction 

 division. The two individuals now fuse, one behaving as a male by 

 sending out a pseudopodium towards the other, and a strong inner 

 cyst forms around the zygote. After a while the latter undergoes 

 binary fission and the two products escape from the cyst. Occasion- 

 ally two individuals enter a jelly cyst together and then either the two 

 gametes of each undergo cross-conjugation with those of the other, 

 or there is one cross-conjugation and the remaining gamete of each of 

 the two original individuals performs parthenogenesis. In fresh and 

 marine waters. 



^ Paedogamy is a kind of autogamy in which not only the nucleus but 

 also its cytoplasm divides and reunites. 



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