56 ZOOLOGY sect. 



frothy appearance. The shell itself, moreover, in this and allied 

 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 

 (megaspheric andmicrospheric) differing from one another in the size 

 of the central chamber, the shape and mode of growth of the suc- 

 ceeding chambers, and the number and size of the nuclei (Fig. 38). 



The reproduction of Foraminifera is mainly by spore-formation, 

 with or without conjugation. The protoplasm has been observed 

 in some to divide into minute masses which may be amoeboid or 

 may be of the nature of flagelluke — each provided with a 

 flagellum. In some cases the flagellulae have been observed to 

 conjugate in pairs. The young may develop shells while still 

 within the shell of the parent or only after becoming free. In the 

 dimorphic Foraminifera there is evidence of the occurrence of an 

 alternation of generations (p. 41) — the megaspheric form alternat- 

 ing with the microspheric, and the latter being developed as a 

 result of a process of conjugation, the former without it (alterna- 

 tion of sexual and asexual generations). 



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 Globigerinse 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 

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



Order 3. — 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," 

 Actinophrys sol (Fig. 39). The body is nearly spherical, and 

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

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

 pseudopods has a firm axis, apparently composed of protoplasm, 



