44 ORIGIN OF THE METAZOA 



colonial forms is not clear. Baker (1948) and Ulrich (1950) 

 suggest that the Cnidosporidia may be degenerate Metazoa but 

 since complex spores are also found in other Sporozoa, e.g. the 

 cysts of gregarines, and since the sporozoite of the Cnidosporidia 

 is very similar in structure to that of Monocystis, it is more likely 

 that they are real sporozoans and not degenerate Metazoa (Fig. 16). 



Under certain conditions the rhizopod Naegleria can aggregate 

 so that the cells being joined by a sticky material form a sheet of 

 tissue (Willmer 1956). Sphaerozoum is another colonial rhizopod 

 — a radiolarian — and in the Mycetozoa there are many forms that 

 show colonial structure at certain stages of their life history. What 

 is of interest here is that colonies such as Dictyostelium have 

 developed a chemical system that keeps the amoebae that go to 

 make up the colony in a unit (Bonner 1949). In general the 

 Rhizopoda tend to form syncytia more easily than they form 

 colonies. On the other hand quite complex multilocular skeletons 

 are found in the Foraminifera, but there is usually only one living 

 cell present. 



From the foregoing account it is evident that the Metazoa could 

 have arisen from the colonial protozoans. There are a large 

 number of colonial Protozoa and many of them show differentia- 

 tion and division of labour amongst the colony. Whether the 

 Metazoa did in fact arise from the colonial protozoans is another 

 matter and we must now consider the alternative theories. 



(2) Origin from a syncytial cell 



This theory suggests that the origin of the Metazoa must be 

 sought from a protozoan that had many nuclei and which later 

 developed membranes separating these nuclei off. The syncytium 

 differs from the colony in that the primary unit is the whole 

 animal and that it later becomes multicellular. In the colony the 

 primary unit is the cell and many of these units come together to 

 form the animal. 



The differences between the syncytium and the colony are not 

 as clear cut as could be desired. In the main they depend upon 

 the absence of cellular boundaries. But what does one say in the 

 case of Volvox or Pheopolykrikos where there is protoplasmic 

 connexion between the cells (Figs. 10 and 11). Is this a multi- 

 cellular animal or a syncytium? 



