The Sarcodina 257 



wall of the preceding chamber becomes a septum in the simpler cases and 

 the old aperture now becomes a foramen joining the two chambers. These 

 foramina were responsible for the name assigned to the group when the 

 Foraminiferida were still considered Mollusca. The foramina in such 

 multilocular species as Planorbulina mediterranensis (82) and Elphidium 

 crispiim (66) are gradually closed by "chitinous" deposits which first 

 appear as rings and then spread across the openings to form "plugs." 

 Periodically, the plugs break loose and are carried out of the test by cyto- 

 plasmic currents. The functional significance of such plugs is unknown. 

 The septa are double in many specialized Foraminiferida, a posterior 

 wall of each new chamber being deposited over the anterior wall of the 

 preceding chamber. The structure may be further complicated by a cana- 

 licular system composed of tubules running through the wall of the test 

 and within the double septa. The canals communicate with the chambers 

 and also open to the outside, independently of the usual pores. 



The endoplasm. The endoplasm contains the nucleus, or the multiple 

 nuclei of mature agamonts, and various types of inclusions, including 

 xanthosomes in many species. In some of the multilocular types, freshly 

 ingested prey and undigested residues tend to be concentrated in the 

 last or the last few chambers. In others, which complete digestion outside 

 the test, the inner cytoplasm may be entirely free from such materials 

 (66). 



Life-cycles. A dimorphic life-cycle involving an alternation of genera- 

 tions was attributed to Foraminiferida in the early work of Lister and 

 Schaudinn. Two adult forms were recognized, a megalospheric type (ga- 

 mont) and a microspheric type (agamont), on the basis of a difference in 

 size of the proloculum. Reproduction of the microspheric adult, by 

 schizogony, results in uninucleate organisms which develop a proloculum 

 larger than that of the parent. At maturity, each megalospheric organism 

 produces gametes. After syngamy each zygote secretes a small proloculum 

 and growth results in a microspheric adult. 



Recent investigations have indicated that this concept is strictly ap- 

 plicable only to certain specialized Foraminiferida which produce flagel- 

 late gametes (82). In other cases, the two forms cannot be distinguished 

 by size of the initial chamber, and occasionally the significance of micro- 

 spheric and megalospheric forms in the life-cycle may appear to be re- 

 versed if only the absolute measurements of the initial chambers are 

 considered. In some of these apparent contradictions, however, the micro- 

 spheric chamber actually is smaller, in proportion to size of the test, than 

 the megalospheric chamber. Life-cycles are now known to vary consider- 

 ably in their details (Fig. 5. 39). Complications include the appearance, 

 in certain species, of two varieties of agamonts, one with a larger pro- 

 loculum than the other. In the terminology of Le Calvez (82), apogamic 

 life-cycles, as observed in Discorhis orbicularis, involve only the sequence 



