156 



THE ANIMAL KINGDOM 



connected with each other through cytoplasmic bridges, by which cell 

 activity can be synchronized. The superficial resemblance of Volvox to 

 the embryonic blastula of metazoans has given this organism a prom- 

 inent place in zoology. 



Most botanists believe that the higher plants evolved from the 

 phytomonads. Many of the noncolonial species, able to swim with 

 ilagella, can also grow upon the bottom as round cells without flagella, 

 in which case they take on a colonial appearance and resemble plants. 

 Further kinship with the higher plants is suggested by certain simi- 

 larities in their chemical structure. 



Choanoflagellates. Most of the strictly animal flagellates are small, 

 uncommon, or are inhabitants of foul water and thus are unpleasant 

 to study. One order, prominent because of their resemblance to the 

 sponges, are the choanoflagellates (order Choanoflagellata, Fig. 8.7). 

 These are sedentary flagellates, attached to the bottom by a posterior 

 stalk. The single flagellum is surrounded by a delicate protoplasmic 

 collar. The flagellum produces a water current over the animal, and 



Desmare^llsL 



Sphaerocca, 



PhalarLstcrium. 



Figure 8.7. Examples of the order Choanoflagellata showing solitary and colonial 

 forms. The matrix holding colonial individuals together is more pronounced than in the 

 Phvtomonadina. 



