PROTOZOA 4I 



in the fresh-water Euglena (Fig. 18). The flagella serve not 

 only for locomotion, but also for procuring food. Undigested 

 substances usually leave the body near the posterior end, but 

 there is no permanent opening like the mouth. 



Reproduction generally takes place by simple division, either 

 during the active condition of the individual or after it has se- 

 creted a cyst about itself; in the latter case division may result 

 in two, but often in several, new individuals. In the colonial 

 Flagellata each individual in some genera divides a number of 

 times and becomes a colony, so that division results in as many 

 new colonies as there were individuals in the old. In other 

 cases, as, for example, in Volvox (Fig. 20), which consists of a 

 hollow sphere, the wall being composed of a single layer of 

 individuals, certain cells enlarge and divide, forming new colo- 

 nies which come to lie inside the hollow sphere ; eventually they 

 are set free by the death and disintegration of the parent colony. 

 In some Flagellata there is a more complex method of repro- 

 duction, which closely resembles sexual ; this is well illustrated 

 by Volvox again (Fig. 20). Some cells in the colony greatly en- 

 large and become macrospores, others divide a large number of 

 times and become microspores ; these two kinds of spores fuse 

 with one another, and then division results in a new colony. 



Order 2. Choanoflagellata 



The Choanoflagellata (Gr. %ogw?7, a funnel) are readily distin- 

 guished by the presence of a protoplasmic collar or cup, which 

 surrounds the base of the flagellum, and both the collar and the 

 flagellum are contractile. There is a single, spherical nucleus 

 and one or two contractile vacuoles. Some are naked, others 

 have a chitinous, cuplike shell, as in some of the Flagellata 

 (Fig. 23), and many are raised on stalks. These animals may 

 live singly (Fig. 23), in branched colonies (Fig. 24), or as irregu- 

 lar groups of individuals (Fig. 23). They occur in fresh and in 

 salt water. Reproduction is generally by longitudinal division, 

 but spore formation attending encystment also occurs. 



Order 3. Dinoflagellata 



The Dinoflagellata (Gr. Slvos, a whirl) generally possess two 

 flageila, one extending freely from the anterior end of the body, 



