I. RHIZOPODA: THALAMOPHORA 



179 



the chief opening of the shell and in the Perforata probably through the 

 pores in the shell wall. They are rarely finger-like (fig. 129); usually 

 they are thread-like, branched and anastomosing (tigs. 17, 118). 



Reproduction is generally by fission, but presents many variations. Only 

 rarely (fresh water Monothalamia) do both animal and shell divide; frequently 

 the protoplasm protrudes from the mouth of the shell, a new shell is formed on 

 the outgrowth and the protoplasm then divides, one of the resulting individuals 

 retaining the old shell. In the marine Polythalamia the following process is 

 general: The polynucleate protoplasm divides into numerous uninucleate 

 'embryos' which frequently, while still within the mother, develop a shell. 



FIG. 130. Protoplasm of Globigerina 

 after solution of the shell, n. nucleus. 



FIG. 131. Young Milinla \vith 

 several nuclei (from Lang). 



A second kind of reproduction leading to a fertilization process appears to be 

 common. Many swarm spores arise in the shells of the Polythalamia. These 

 fuse in pairs with each other. Both of these reproductions alternate with e;u h 

 other, and with them is often connected a dimorphism of the individuals. The 

 progamic generation, leading to the formation of gametes is distinguished by 

 the long persistence of the chief nucleus and often by the structure of the shell 

 (large central chamber, megasporic generation) of the metagametes arising 

 from fertilization (polynucleate, microspheric gametes). A corresponding 

 alternation has been observed in the Monothalamia. 



Sub Order I. MONOTHALAMIA. Mostly fresh-water. They have one- 

 chambered shells of chitin or silica, often strengthened by foreign bodies. Con- 

 tractile vacuole usually present. Pseudopodia lobose or filiform, branched or 

 simple. A. Forms with finger-form pseudopodia: Arcclla,* (Jiiadrulu* (fig. 

 129); Difflttgia* These forms are merely shelled Amoeba: and are frequently 

 referred to the Lobosa. B. Forms with branching and anastomosing filiform 

 pseudopodia. Euglypha,* Groinia (fig. 17). Sub Order II. POLYTHAL- 

 AMIA. Exclusively marine; many-chambered shells. Thick beds of rock 

 like the chalk, nummulitic limestone, and green-sand are largely foraminiferal 

 in origin. The living species have an average diameter of about i mm. Rarely 

 species have a diameter of several centimeters (Psammonyx i'ulf<iiiiciis, 5-6 cm.). 

 The fossil nummulites reach a diameter of 10 cm. A. IMPERFORATA. Shell 



