n PHYLUM PROTOZOA 59 



rocks, such as the White Chalk (Cretaceous period) and the 

 Nummulitic limestone (Eocene), being largely made up of them. 



ORDER 4. HELIOZOA. 



General Structure. The Heliozoa are at once distinguished 

 from the preceding groups by the character of their pseudopods, 

 which have the form of stiff filaments radiating outwards from 

 the more or less globular cell-body, presenting very little move- 

 ment beyond the characteristic streaming of granules, and not 

 uniting to form networks. 



One of the simplest forms is the common " Sun-animalcule," 

 ActinopJirys sol (Fig. 40). The body is nearly spherical, and 

 contains a large nucleus and numerous vacuoles, one of which, 

 near the surface, is contractile. Each of the stiff radiating 

 pseudopods has a firm axis, apparently composed of protoplasm, 

 which is traceable through the general protoplasm as far as the 

 nucleus. Living organisms are devoured in much the same way as 

 in Amoeba : each is ingested along with a droplet of water, and is 

 thus seen, during digestion, to lie in a definite cavity of the proto- 

 plasm, called a food-vacuole. If the organism be small, processes of 

 the protoplasm are developed, and surround and engulf it. If it 

 be larger, several pseudopods are applied to it, their axial fibres 

 becoming absorbed, and their substance envelops it, enclosing it in 

 a vacuole. The animal can fix itself by means of its pseudopods, the 

 ends of which become viscid, and it is able to crawl slowly by their 

 means. Sometimes it floats freely in the water, and it possesses the 

 power of rising or sinking by some unknown means. 



Actinosphcerium (Fig. 41, A), another fresh-water form, is more 

 complex. The protoplasm is distinctly divided into a central mass, 

 the medulla or endosarc (B, med.), in which the vacuoles are small, 

 and an outer layer, the cortex or ectosarc (cort.), in which they are 

 very large. There are numerous nuclei (nu.) and chromatophores 

 (chr.), the latter coloured green by chlorophyll, the characteristic 

 pigment of green plants. The axial filaments of the pseudopods 

 are in relation each with one of the nuclei. 



Many genera form colonies. Numerous zooids may be united 

 by bridges of protoplasm into an open network, or the connecting 

 bridges may be shorter and the zooids more numerous, giving the 

 colony a more compact appearance. 



There may be two nuclei of different kinds one centrosome-like 

 body in the centre with the filaments of the pseudopods radiating 

 out from it, and the other a more superficially situated nucleus of 

 ordinary character : the former is derived from the latter. 



Transitional stages occur between the naked genera already re- 

 ferred to and forms with a distinct skeleton. Sometimes the body 

 simply surrounds itself with a temporary gelatinous investment 



