ORIGIN OF THE METAZOA 



37 



(A). Chlamydomonas 



Fig. 8. (A) Palmella stage of Chlamy- 

 domonas. During this stage the proto- 

 zoan divides but the cells remain 

 together enclosed in a gelatinous case. 

 The stage is " multicellular " though 

 it is in fact a resting stage in the life 

 history of the protozoan. (FromGrasse 

 after Goroshankin.) 



Haematococcus 



Fig. 8. (B) Palmella stage of 



Haematococcus. (From Grasse 



after Wollenweber.) 



alike and at reproduction each of them divides and forms gametes. 

 A more complex colony is that of Eudorina in which there are 

 sixty-four individuals (Fig. 9). 



Pleodorina illinoiensis and Pleodorina californica show further 

 stages in the development of the colony in that a group of cells 

 become differentiated from the others and they are unable to take 

 part in reproductive activities. There are four somatic cells in 

 Pleodorina illinoiensis and thirty-two in Pleodorina californica. 

 The soma is even more developed in Volvox, where the majority 

 of the cells are unable to take part in the reproductive activity. The 

 cells beat their flagella in a co-ordinated manner so that the 

 colony can be regarded as having an antero-posterior axis and a 

 dorso- ventral axis. The dorsal cells are slightly larger than the 

 ventral cells and the colony during locomotion moves slowly 

 through the water and does not turn over and over (Fig. 10). 



