RECENT MEMOIRS ON FRESHWATER RHIZOPODA. 295 



Heliozoa may be demonstrable — demonstrated, at present, it 

 is not. 



The Heliozoa, then, are an independent Class, and the 

 following Avould appear to be their characteristics. 



They are unicellular, rarely (by multiplication of nuclei) mul- 

 ticellular (or multinucleated ?) organisms. Their fundamental 

 form is that of a sphere, whilst a very few are fixed by a 

 stipes. The protoplasm, of which alone the soft part of the 

 body is wholly composed, is in the greater number of forms 

 differentiated into an endosarc and ectosarc. 



The limitation of these layers is more or less pronounced ; 

 in Actinophrys sol they jDass imperceptibly one into the 

 other ; in Actinosphserium the modification is complete ; the 

 limits seem most sharply marked off in Acanthocystis, 

 Heterophrys, &c., but even in them it is a differentiation of 

 the protoplasm, not a special membrane, which determines 

 the distinctness of their contour. 



In the endosarc constantly lie the nuclei ; if the nucleus, 

 as mostly, is single, it is more or less excentric ; if there be 

 numerous nuclei, they are irregularly scattered. 



The ectosarc is characterised by the possession of contrac- 

 tile vacuoles (though not as yet demonstrable in all species). 

 They mostly occur at given places, and are variable in 

 number. In Actinophrys and others occur likewise simple 

 vacuoles, which are more highly developed in the ectosarc, 

 but may extend even into the endosarc. 



The pseudopodia, serving, as they do, both to the capture 

 of nutriment and to locomotion, are constantly thin and fili- 

 form, and originate all round the superficies of the globular 

 body ; they frequently reach a length several times exceeding 

 the diameter of the body, are sometimes homogeneous, 

 and sometimes granules pass along them slowly up and 

 down. Ramifications or anastomoses sometimes, but do not 

 usually occur, to some extent possibly due to their radial 

 arrangement forbidding the opportunity for their mutual 

 contact. 



A speciality of the pseudopodia of many Heliozoa is the 

 differentiation of their substance into an axis and cortical 

 investment, to which, in Actinosphserium, Max Schultze was 

 the first to draw attention ; this extremely delicate axis 

 reaches the length of the pseudopodium and passes down 

 at least as far as the endosarc ; it consists of a homogeneous 

 substance dissolved by acetic acid. The authors regard this 

 axis as a strengthening apparatus for the pseudopodia, and 

 probably only due to a condensation of the protoplasm. They 

 regard GreefTs comparison of it to the spines penetrating into 



