182 ] The Classification of Lower Organisms 



somes. The chromosomes are numerous, long, and slender; the mitotic figures re- 

 semble those of Pyrrhophyta. Reduction of the chromosome number is said to be 

 effected by a single nuclear division, the last one before the formation of gametes, 

 which cuts the chromosome number of Patellina from 24 to 12, and that of Spirillina 

 from 12 to 6. Before they reach this stage, the megalospheric individuals have 

 gathered themselves in clusters of two or more within cyst walls consisting of secreted 

 gelatinous matter and scraps from the neighborhood. Gametes from one individual 

 are unable to unite with each other. The gametes are amoeboid, positively without 

 flagella. In Discorbis and Cymbalopora, however, Myers (1943) observed the produc- 

 tion of biflagellate gametes. 



Le Calvez (1950) has cleared up various questions raised by earlier studies. Some 

 forms, as Discorbis orbicularis, appear to lack a sexual cycle. Patellina and Spirillina 

 produce amoeboid gametes 40-50[.i in diameter. Most rhizopods produce biflagellate 

 gametes 1.5-4[i long. Le Calvez found the flagella definitely unequal. In Discorbis 

 mediterranensis he showed that the megalospheric individuals are of two mating 

 types. Earlier zoologists, apparently misled by familiarity with the normal life cycle 

 cf animals, had identified meiosis as occurring at the time of gametogenesis; it is the 

 fact, on the contrary, that it occurs in the last two nuclear divisions in the micro- 

 spheric individuals. The megalospheric and microspheric stages of rhizopods are 

 respectively haploid and diploid, like the gametophytes and sporophytes of plants. 



With the possible exception of some of the one-chambered fresh water forms, the 

 rhizopods are clearly a natural group. The fresh water forms appear to intergrade 

 with organisms which Pascher identified as chrysomonads. 



The shells of dead rhizopods may under appropriate conditions be preserved 

 through geologic ages. Natural chalk consists of shells of Textularia mixed with coc- 

 coliths. Certain forms of limestone consist chiefly of shells of Miliola. Certain fossil 

 rhizopods have long been known as indicators of division of geologic time. Since about 

 1917, it has been found that the whole group offers one of the beautiful illustrations 

 of evolution as related to geologic time: the shells of rhizopods found under magnifi- 

 cation in a particular stratum serve promptly and precisely to identify it. The services 

 of experts on "Foraminifera" have acquired a high economic value in the petroleum 

 industry: these experts have found themselves promoted from the status of pure 

 biologists to that of economic geologists. 



Among some eleven hundred genera which have been published, Galloway (1933) 

 maintains 542. Of the number of species one can only say that it is a matter of 

 thousands, but probably not many tens of thousands. Economic micropaleontologists 

 find themselves dealing with great numbers of forms which are slightly, yet signifi- 

 cantly, distinct. They find it expedient not to name these, but to identify them by 

 comparison with available collections. 



Some of the marine and fossil forms are similar, on a small scale, to the animal 

 Nautilus, and Linnaeus placed some of them in that genus. Montfort and Lamarck 

 treated them as several genera of mollusks. In first distinguishing these organisms as 

 the order Foraminiferes of class Cephalopodes, d'Orbigny intended to contrast them 

 with Nautilus, in whose shells a series of chambers arc connected, not by holes (fora- 

 mina) but by cylindrical tubes. Dujardin ( 1835) found that his Rhizopodes are with- 

 out definite organs. Their shells enclose a clear semiliquid substance; their apparent 

 tentacles are merely temporary structures, formed of this substance, thrust forward 

 in the direction of the movement of the shell and withdrawn as it advances. Dujardin 

 named this substance sarcode; it is, of course, the same which has since been called 



