42 INVERTEBRATE ZOOLOGY 



closed at one pole and open at the other. In form, the shell 

 ranges from a simple, single chamber (Monothalmia) to complex 

 series (Fig. 26 B) of spirally arranged chambers (Polythalmia). 

 Growth of the latter is through the periodic formation of new 

 chambers of increasing dimensions. Foramina serve to com- 

 municate between the various chambers of the shell, all of which 

 are filled with protoplasm. As the protoplasm increases in bulk, 

 it protrudes beyond the limits of the old shell and a larger chamber 

 is added to the chambers previously formed. Pseudopodia of 

 different types protrude through either a single opening or numer- 

 ous small apertures. In some of the smaller forms, the shell is 

 almost wholly of organic material formed by the protoplasm. 

 More commonly, silicious or calcareous matter is also present, and 

 frequently foreign bodies are incorporated directly into the sub- 

 stance of the shell. Chalk deposits, the nummilitic limestones, 

 and various other sedimentary rocks have been formed largely 

 of the fossil shells of Foraminifera. 



Members of the suborder Polythalmia (Fig. 26) are exclusively 

 marine. The shells, which are of extremely diverse types, may 

 be either pure lime or partly of organic matter. Those of some 

 fossil forms are relatively immense in size {Psammonyx vulcanis, 

 5 to 6 cm.). Some idea of the rate of reproduction of these forms 

 may be gained from the fact that chalk deposits many feet in 

 thickness are composed almost exclusively of their shells (Bilo- 

 culina). In Norfolk, England, the chalk measures have an 

 average thickness of 1,450 feet. It is reported that nearly 

 50,000,000 square miles of ocean bed is covered with a globigerina 

 ooze. 



The slime molds are frequently considered as fungi, but though 

 they are border-line forms between plants and animals the 

 structure and development of the vegetative body gives proof of 

 close relationship with other rhizopods. The young stages are 

 small amoeboid forms which may, by direct transformation, 

 develop a flagellum. Transition between these two stages is 

 accomplished very readily. Both reproduce by ordinary fission 

 and each has the power of ingestion of solid food material and 

 absorption of organic fluids. The flagellate form may become a 

 microcyst from which either the flagellate (myxoflagellate) or the 

 amoeboid (myxamoeba) form may emerge. 



Logs, stumps, and fallen leaves in moist woodland are especially 

 favorable places for discovering sporangia of slime moulds in all 



