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 sponges (p. 629). Moreover, there is con- 

 siderable other evidence that indicates that 

 the flagellates provided a main line of ascent 



COLLAR 



TRACHELOMONAS 



Fig. 32-2. Protozoa with flagella: 

 Class Mastigophora. Note that 

 some forms are colonial. Also note 

 the "collared" species, a Choano- 

 flagellate. 



