434 



EVOLUTION OF ANIMALS 



Part V 



Green volvox of the ponds 



Fig. 21.7. A, Volvox, a colony of thousands of cells, most of them with two fla- 

 gella, a red eyespot, contractile vacuole, and chlorophyll. Strands of protoplasm 

 unite the asexual cells and make them physiologically continuous. Certain of the 

 cells reproduce by division. Certain cells in some colonies enlarge and become 

 female sex cells; in others certain cells divide and become male sex cells. These 

 fuse with the large cells in the female colonies and form daughter colonies which 

 remain for a time within the parent colony. B, a detailed view of the surface of 

 Volvox highly magnified showing the protoplasmic connections between the cells. 

 (B, courtesy, Hyman: The Invertebrates, vol. 1. New York, McGraw-Hill Book 

 Co., 1940.) 



brates feed upon them especially along the coasts. Epidemics of human food 

 poisoning have been traced to eating mussels (Mytilus californicus) which had 

 fed upon a species of Gonyaulax that produces an alkaloid poison (Fig. 21.8). 

 The "red tide" that came in along the Florida coast in 1947 brought poisonous 

 dinoflagellates in untold numbers and tons of dead fishes were strewn for many 

 miles upon the shore. 



ZOOFLAGELLATES 



Definitely animal-like, zoofiagellates do not contain chlorophyll, and usually 

 have one or two flagella. They may be solitary or colonial, and many are para- 

 sitic. The collar-flagellates (choano-flagellates) that live mainly in fresh water, 

 have transparent protoplasmic collars. The single flagellum swings forth from 

 within the collar and draws food against the cell along with the currents of 

 water that it creates (Fig. 21.9). In the sponge-like colonial Proterospongia, 

 the individuals are embedded in a blob of clear jelly; collared cells protrude 

 from the surface and collarless ameboid ones migrate into the interior of the 

 jelly. Collared cells are very characteristic of sponges, and Proterospongia 

 appears like a hesitant step in an evolution toward a structure similar to 

 sponges. 



