Phylum Protoplast a [181 



The individual rhizopod originates as a minute amoeboid cell which secretes a 

 shell from which the pseudopodia project. In the fresh-water forms, each protoplast, 

 after moderate growth, divides into two, one of which retains the original shell while 

 the other secretes a new one. In some of the marine forms, the original protoplast, 

 having a cylindrical or irregular shell, enlarges this as it grows. In the great majority 

 of the group, the original shell, called the proloculus, is of definite size and form and 

 has a constricted orifice. When the protoplast reaches a certain stage, it expands, pro- 

 trudes from the orifice, and secretes an extension of the shell in the form of a second 

 chamber. In some few examples, the second chamber is the final one, being capable 

 of indefinite extension. But again in the great majority, the second chamber, although 

 diff'erent from the proloculus, resembles it in being definite in form and in having a 

 constricted orifice. After further development, the protoplast again protrudes through 

 the orifice and secretes a third chamber, generally of the same form as the second, 

 though often larger. Repetition of this process produces macroscopically visible 

 bodies. Even though becoming a centimeter or more in diameter, the individuals 

 continue to be single cells. 



As a result of different patterns of growth, the developed shells are of highly varied 

 forms, linear, globular, or coiled in one plane; trochoid or rotaloid, that is, helical, 

 of the form of a low cone; of the form of high cones; or screw-like, with the chambers 

 in fixed longitudinal rows. The grov/th pattern may change during the life of the 

 individual. There are apparently degenerate forms, simple or irregular. It is highly 

 probable that some of the forms have evolved repeatedly. 



The shells may be of gelatinous material or of chitin, without or with imbedded 

 grains of sand. Exceptionally, they are siliceous. They are sometimes of crystallized 

 calcium carbonate with imbedded grains of sand. In the bulk of the group they consist 

 of crystallized calcium carbonate without foreign matter, and are of either of two 

 t>'pes of texture: vitreous, that is, hyaline, and punctured by numerous pores a few 

 microns in diameter; or porcellanous, white by reflected light and amber by trans- 

 mitted light, and with no perforations except the proper orifices. In fossil shells, other 

 textures than these may occur; it is supposed that these are products of modification 

 during preservation. Some of the textures, like some of the forms, are believed to 

 have evolved repeatedly. 



Most rhizopods occur in two forms which are most readily distinguished by the 

 size of the proloculi. This was first pointed out by Munier-Calmas, 1880; who, 

 jointly with Schlumberger, 1885, designated the smaller and larger proloculi re- 

 spectively microsperes and megaspheres. Lister (1895), by study in culture of Elphi- 

 dium crispiirn [Polystomella crispa Lamarck), showed that the two forms are alter- 

 nate generations. He observed that the microspheric cells become multinucleate 

 during growth, while the megalospheric cells remain uninucleate until just before 

 reproduction. The reproduction of the megalospheric cells is by release of numerous 

 minute biflagellate cells. 



Schaudinn (1902) confirmed much of what Lister had observed. He was mistaken 

 in describing nuclear division (except just before the production of the swimming 

 cells) as non-mitotic; and correct in identifying the swimming cells as gametes. 

 Winter (1907) observed a similar life cycle in Peneroplis, but described the gametes 

 as having solitary flagella. 



Myers" (1934, 1935, 1936), dealing with Patellina and Spirillina, described the 

 details of mitosis. This takes place within an intact nuclear membrane, and is com- 

 pleted by its constriction. The spindle is blunt-ended; there is no evidence of centre- 



