356 The Animal Kingdom 



Each colony of Volvox is a ball-shaped structure whose center is 

 filled with a jellylike fluid. The wall of the colony is formed of thou- 

 sands of tiny cells, each possessing two flagella, a nucleus, chloroplast, 

 red stigma or eye spot, and a contractile vacuole. The individual cells 

 are connected to one another by protoplasmic strands or bridges. The 

 beating of the flagella rolls the colony through the water with the en- 

 tire colony demonstrating positive phototaxis. In most colonies, the 

 cells forming the outer wall are strictly vegetative, while the reproduc- 

 tion of the colony is taken care of by other specialized cells. 



During favorable conditions, Volvox reproduces partheno- 

 genetically. Certain surface cells migrate toward the interior of the 

 colony and lose their flagella. By repeated divisions, these form daugh- 

 ter colonies. After several are developed, the parent colony ruptures 

 and the daughter colonies escape. At times bisexual reproduction also 

 occurs. Certain surface cells which have lost their flagella enlarge 

 to become female sex cells or macrogametes. Other cells in the same 

 colony, or perhaps in another colony, divide numerous times to pro- 

 duce a flat bundle of microgametes or male sex cells. The gametes 

 fuse to produce a zygote which secretes a heavy protective, spiny wall 

 about itself. These zygotes are able to withstand adverse conditions 

 such as cold and drying. When favorable conditions return, they 

 develop into new colonies. 



Other Colonial Flagellates. — It is interesting that Volvox not 

 only shows the beginnings of cellular differentiation, but also super- 

 ficially resembles the blastula of metazoan embryology. Another 

 colonial flagellate, Pandorina (Fig. 112,G), resembles late cleavage 

 stages; while Gonium (Fig. 112,7) with 4 to 16 cells resembles early 

 cleavage. Of course, it is well known that the higher forms did not 

 evolve through these modern specialized protozoans, but they may 

 give us a hint as to the method followed in the development of the 

 metazoans. 



Other Phytomastigina. — There are many different types of 

 phytomastigina that inhabit both fresh and salt waters. One group is 

 that of the dinoflagellates (Fig. 113, E,F, H). These are very abundant 

 in the ocean and not infrequent in fresh waters. Many species have a 

 covering of irregular plates and typically they possess two flagella, one 

 transverse and one longitudinal. Each lies in a groove with the longitu- 

 dinal flagelluni imparting forward motion and the transverse one giving 

 a rotating motion. They are one of the more abundant organisms present 



