FOR A MINIFERA 195 



The last chamber has one or more apertures of variable size, 

 through which the cytoplasm extends to the exterior as myxo- 

 podia. 



The food of the Foraminifera consists mostly of diatoms and 

 algae. Pelagic forms are known to capture other Protozoa and 

 microcrustaceans. 



All species of Foraminifera manifest a more or less distinct 

 tendency toward a dimorphism : the smaller megalospheric form 

 with a large proloculum and the larger microspheric form with a 

 small proloculum (Fig. 73). The former is usually much more 

 numerous than the latter. The microspheric form is multinu- 

 cleate, in which the nuclei are scattered without apparent order, 

 and vary in size proportionately with the size of the chambers. 

 As the animal grows the nuclei increase in number. Around 

 each of them a small island of cytoplasm becomes condensed 

 (c). The uninucleate bodies thus formed leave the parent body, 

 and each secretes around itself a test which is much larger than 

 the proloculum of the parent individual {d, e). To this prolocu- 

 lum, new chambers are added one by one, as the animal grows 

 (/) and at the same time the single nucleus shifts its position, so 

 that the latter is almost always in the middle chamber. As the 

 organism grows, endosomes appear in increasing numbers in the 

 nucleus which multiplies finally into many nuclei {g). Each of 

 these nuclei becomes the center of a swarmer. The swarmers 

 leave the parent test and undergo fusion in pairs to produce 

 zygotes {h-j). The zygote secretes a test around itself (a) and 

 forms first a small proloculum, to which are added many cham- 

 bers {h). This is the microspheric form. Thus here one sees an 

 alternation of asexual and sexual generations. In some forms the 

 microspheric generation appears to be unknown. 



More than three hundred genera of extinct and living Fora- 

 minifera are now known. Cushman distinguished forty-five 

 families. The present work follows Cushman in recognizing and 

 differentiating forty-four families, and lists one genus as an 

 example for each, but places the Allogromiidae in the order 

 Testacea (p. 232). 



Test entirely or in part arenaceous 



Test single-chambered or rarely an irregular group of similar cham- 

 bers loosely attached 



