

PHYLUM PROTOZOA 55 



forms is provided with numerous delicate, hollow, calcareous spines 

 (sp.), which are only to be seen in perfect, freshly-caught specimens. 



Many Foraminifera exhibit the phenomenon of dimorphism : 

 the individuals of a single species occur under two distinct forms 

 differing from one another in the size of the central chamber, 

 the shape and mode of growth of the succeeding chambers, and 

 the character of the nuclei. 



The reproduction of the Foraminifera is very imperfectly 

 known ; but in some forms the protoplasm has been observed to 

 divide into flagellulce or swarm-cells, minute masses of protoplasm, 

 each provided with a flagellum : usually these are of uniform size, 

 but in some cases large and small spores are produced. In some 

 species young forms, provided with a shell, are formed in the 

 terminal chamber of the adult. 



Distribution. Gromia, Microgromia, and a few other forms are 

 found in fresh- water : one species has been found in damp earth, 

 but the great majority of the Foraminifera are marine, some being 

 pelagic, i.e. occurring at or near the surface of the ocean, others 

 abyssal, i.e. living at great depths. In the Atlantic, large areas 

 of the sea-bottom are covered with a gray mud called Globigerina- 

 ooze from the vast number of Globigerime contained in it. 



From the palaeontological point of view, the Foraminifera are a 

 very important group. Remains of their shells occur in various 

 formations from the Silurian period to the present day, certain 

 rocks such as the White Chalk (Cretaceous period) and the Num- 

 mulitic 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, present- 

 ing very little movement beyond 

 the characteristic streaming of 

 granules, and not uniting to form 

 networks. 



One of the simplest forms is 

 the common " Sun-animalcule," Ac- 

 tinophn/s sol (Fig. 37). The body 



2 J , i - & i i , - F IG - 37. Actinophrys sol. a. axial 



IS nearly Spherical, and Contains filaments of pseudopods ; n. nucleus ; 



a large nucleus and numerous SSffS3LJ!3SSL& > ' 

 vacuoles, some of which, near the 



surface, are contractile. Each of the stiff, radiating pseudopods 

 has a delicate axis, which is traceable through the protoplasm 



