. Principles of modern biology. Biology. 626 - Heredity and Evolution ish mud (foraminiferous ooze), which gradu- ally may undergo transformation into chalk. One such deposit is represented by the beau- tiful "white cliffs of Dover" in which the shells of the original organisms can easily be identified microscopically. And even today foraminiferans and radiolarians are very abundant in the sea. Accumulations of the Radiolaria, on the other hand, usually undergo metamorphosis into siliceous rock (for example, flint). Today, indeed, a study of the distribution of the ancient marine Sar
. Principles of modern biology. Biology. 626 - Heredity and Evolution ish mud (foraminiferous ooze), which gradu- ally may undergo transformation into chalk. One such deposit is represented by the beau- tiful "white cliffs of Dover" in which the shells of the original organisms can easily be identified microscopically. And even today foraminiferans and radiolarians are very abundant in the sea. Accumulations of the Radiolaria, on the other hand, usually undergo metamorphosis into siliceous rock (for example, flint). Today, indeed, a study of the distribution of the ancient marine Sarcodina, especially the Fora- minifera, is very useful to petroleum geolo- gists who are searching lor new oil fields. Also among the Sarcodina are a few para- sitic species. One of these, Endamoeba histo- lytica (see Table 32-1), is the causative agent of amoebic dysentery, a rather serious human ailment. The Mastigophora, or Flagellata. (Fig. 32-2). The flagellates form an extremely var- ied group. Some, like Euglena (Fig. 32-2) and Volvox (Fig. 32-2), possess definite chloro- plasts and can be classified either as plants or animals. Most are truly unicellular, but a large group, including Eudorina, Volvox, and related species, form colonial aggregates of very definite pattern. Such colonies provide a key to how multicellular organisms may have evolved. Indeed, some of these colonies (for example, Volvox) display a clear differ- entiation between the germ and the somatic types of cells. The germ cells of Volvox, as in higher animals, are reproductive cells, which retain the potential of transmitting chromosomes to the next generation. The somatic or body cells, on the other hand, are the nonreproductive cells, in the wall of the colony, and cannot transmit chromosomes to the next generation. Some flagellates possess collarlike struc- tures surrounding the origins of the flagella (Fig. 32-2). Such cells are strikingly similar to the collar cells (Fig. 16-2) of the Porifera, or
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