PHYLUM PROTOZOA 



47 



lished. All of the cells are not alike, since some of them, the germ 

 cells (Fig. 27, S and 9 ) are able to produce new colonies, but 

 others, called somatic or body cells, have no reproductive power. 

 Some of the germ cells, the parthenogonidia, grow large, divide 

 into many cells, drop into the center of the mother colony, and 

 finally escape through a break in the walk Other germ cells ( S ) 



produce by division a great number of 

 minute microgametes or spermatozoa, 

 and still others grow large, becoming 

 macrogametes or eggs ( 3 ) . The eggs 



Fig. 28. — Monosiga, 

 a Choanoflagellate. 

 c, collar; c. vac, contrac- 

 tile vacuole; fl, flagel- 

 lum ; nu, nucleus ; 

 5, stalk. (From the 

 Cambridge Natural His- 

 tory, after Kent.) 



Fig. 29. — Proierospongia harckeli, a 

 colonial Choanoflagellate. a, ameboid 

 cell; b, a cell dividing; c, cell with small 

 collar; s, jelly. (From the Cambridge 

 Natural History, after Kent.) 



are fertilized by the spermatozoa, and, after a resting stage, 

 develop into new colonies. 



Order 2. Choanoflagellata. — Mastigophora with a con- 

 tractile protoplasmic collar from the bottom of which extends 

 a single flagellum. Examples: Monosiga (Fig. 28), Proiero- 

 spongia (Fig. 29). 



Order 3. Dinoflagellata. — Mastigophora with two flagella, 

 one at the anterior end, the other passing around the body, 

 often in a groove. Examples: Peridinium (Fig. 30), Ceratium. 



