FORAMINIFERA 395 



The form of the shell varies greatly. In some there is only one 

 chamber composed of a central body and radiating arms which repre- 

 sent the material collected around the pseudopodia, as in Rhabdam- 

 mina (Fig. 185, a) , or of a tubular body alone, as in Hyperammina (Fig. 

 185, d). The polythalamous forms possess shells of various spirals. 

 The first chamber is called the proloculum which may be formed 

 either by the union of two swarmers or by asexual reproduction. The 

 former is ordinarily small and known as the microspheric proloculum, 

 while the latter, which is usually large, is called the megalospheric 

 proloculum. To the proloculum are added many chambers which 

 may be closely or loosely coiled or not coiled at all. These chambers 

 are ordinarily undivided, but in many higher forms they are divided 

 into chamberlets. The chambers are delimited by the suture on the 

 exterior of the shell. The septa which divide the chambers are per- 

 forated by one or more foramina known as stolon canals, through 

 which the protoplasm extends throughout the chambers. The last 

 chamber has one or more apertures of variable sizes, through which 

 the cytoplasm extends to the exterior as pseudopodia. The food of 

 Foraminifera consists mostly of diatoms and algae, though pelagic 

 forms are known to capture other Protozoa and micro crustaceans. 



All species of Foraminifera manifest a more or less distinct tend- 

 ency toward a dimorphism : the megalospheric form has a large pro- 

 loculum, is uninucleate and is relatively small in size ; while the micro- 

 spheric form possesses a small proloculum, is multinucleate, and is 

 large. In addition, there is a difference in the direction of rotation of 

 spiral chambers of tests in some species (Myers). For example, in 

 Discorhis opercularis, the microspheric form has clockwise rotation 

 of the chambers, and the megalospheric form shows counterclock- 

 wise rotation. The megalospheric forms are said to be much more 

 numerous than the microspheric forms, especially in pelagic species. 

 It is possible that, as Myers (1938) pointed out, the flagellate gam- 

 etes are set free in open water and have a minimum of opportunity 

 for syngamy. 



Lister (1895) observed the development of the megalospheric 

 form in Elphidium by asexual reproduction from the microspheric 

 form. He noticed flagellated swarmers in megalospheric tests and 

 considered them as gametes which through syngamy gave rise to 

 microspheric individuals. Recent studies by Myers (1935-1940) 

 confirm the correctness of this view, except that in some species the 

 gametes are amoeboid. In Spirillina vivipara (Fig. 184, A, 1-5) the 

 mature microspheric form (1) which measures 125-1 52/i in diameter, 

 becomes surrounded by an envelope composed of substrate debris 



