THE HELIOZOA 15 



Proteomyxa (cf. p. 6), no axial rod to the pseudopodium has been 

 discovered. A skeleton may be present or absent ; when present 

 it is generally siliceous, though it may be in part chitinous (Actino- 

 lophus), or composed of a jelly whose chemical composition is 

 unknown (Heterophrys\ or built up of foreign particles (Elaeorhanis). 



Hertwig and Lesser (7), in a memoir which established the 

 main lines of the modern classification of the group, included only 

 those higher forms whose characters have been indicated, giving a 

 conception of the Heliozoa both logical and in many ways con- 

 venient ; but such a treatment neglects a singularly perfect series 

 of forms, the higher members of which, such as Nuclearia (Fig. 8, E, 

 p. 10), closely resemble undoubted Heliozoa, .while from these we 

 may pass step by step to such forms as Monobici' or Vampyrella 

 (Figs. 4 ; G (5), pp. 4 and 7), which are probably more nearly allied 

 to the Mycetozoa than to the typical Heliozoa. We have here, in 

 fact, a case such as often occurs in which different types of structure 

 and life-history are connected by a series of intermediate forms so 

 gradual that any attempt to define the limits of either must fail. 

 Under these circumstances, the limits assigned to one or other group 

 in a descriptive classification depend merely on convenience ; the 

 only point of importance is to frame the classification in such a way 

 that it shall not disguise the real continuity of the forms described. 

 For this reason, most modern writers, while recognising the great 

 value of the conception formulated by Hertwig and Lesser, have so 

 enlarged it as to include among the Heliozoa a number of transi- 

 tional genera (p. 6). 



For the sake of convenience, the forms that are included in the 

 Heliozoa in this article are those in which one or more definitely 

 formed nuclei are present during the vegetative phases of life, 

 together with those genera which seem to have the closest zoological 

 relation to them although their nuclei are not known. The 

 genera that are sometimes classified with the Heliozoa, mainly on 

 the ground that they have stiff radiating pseudopodia, but which 

 afford some reasons for believing that their nuclei are dissipated 

 during the vegetative phases of life, are placed with the Proteomyxa 

 (see p. 6). 



It will be convenient to consider first the structure of the more 

 highly specialised forms to which Hertwig and Lesser proposed 

 that the name Heliozoa should be restricted, and to discuss the 

 transitional genera afterwards. 



The characters of the more specialised Heliozoa may be illustrated 

 by describing Actinophrys sol, the common freshwater species already 

 mentioned. The body is spheroidal and minute, rarely exceeding 

 0*05 mm. in diameter ; in a healthy undisturbed individual numerous 

 stiff pseudopodia, each considerably longer than the diameter of the 

 body, radiate from the surface. The body itself is divided into a 



