40 



ORIGIN OF THE METAZOA 



These examples are merely illustrative phases of the develop- 

 ment of the colonial habit. There is no evidence that Eudorina 

 gave rise to Pleodorina or Volvox. 



Further examples of the development of colonial stages are 

 found in the dinoflagellates. Though some forms such as 

 Gymnodinium are solitary, others such as Ceratium at times may 

 form long chains of individuals joined together in a temporary 

 manner. Polykrikos is of interest since it shows a more permanent 

 attachment. Polykrikos schwartzi usually contains four nuclei and 

 a series of associated sets of flagella. It possesses cnidocysts which 

 are manufactured within the cell and are used in catching prey. 

 There is also a cytoplasmic connexion between the units of the 

 Polykrikos colony. Thus in the related genus Pheopolykrihos 

 beauchampi when the animal is touched it contracts up and 

 clearly shows that there are four units in interconnexion 

 (Fig. 11). 



(A). Normal Animal 



(B). Diagram of shape of animal aftertactilestimulation 



Fig. 11. Colonial dinoflagellates. The dinoflagellate Pheopolykrikos 

 is clearly made from four dinoflagellate units. When it is touched it 

 changes its form (B) and the four units can be distinguished. (From 



Grasse after Chatton.) 



