46 ORIGIN OF THE METAZOA 



Baker (1948) suggested that since in his definition a cell is "a 

 mass of protoplasm largely or completely bounded by a membrane 

 and containing within it a single nucleus formed by the telophase 

 transformation of haploid or diploid set of anaphase chromosomes," 

 that the Ciliophora and the Radiolaria are not cells. They contain 

 more than one nucleus and therefore are syncytia. Other syncytia 

 are found in the Flagellata (Calonympha, Giardia), Rhizopoda 

 (Sappinia, Plasmodiophora), Sporozoa (Myxobolus) and Ciliophora 

 (Paramecium). 



The advantage of deriving the metazoan from a syncytial 

 protozoan instead of a multicellular one is that in a syncytium such 

 as Calonympha or Opalina the animal has an already established 

 symmetry and an antero-posterior axis. All it has to do is super- 

 impose cell walls on the established pattern. In the development 

 of the multicellular form from the colonial pattern one has a series 

 of units each with an already established axis and these axes have 

 to be amalgamated and altered till the cells form a single unit. 



This view of the syncytial origin of the Metazoa is supported 

 by de Beer (1954), who writes, " there are the gravest objections 

 to the view that the Metazoa were evolved by aggregation of 

 separate protozoan individuals. This may have happened in the 

 sponges and, indeed, is the most likely explanation for the lack of 

 co-ordination, integration and individuality found in those 

 animals. One of the most important features in the acquisition of 

 individuality in organisms is axiation and integration throughout 

 the body. The only way in which this can be imagined as having 

 occurred in the transition from Protozoa to Metazoa is by means 

 of internal subdivision of the protozoan cell, by cellularisation. 

 Nor is it difficult to imagine how this might have been brought 

 about, since there are Protozoa such as the Ciliate Infusoria, 

 Haplozoa and some Sporozoa which possess many nuclei, and it 

 would only be necessary to separate these by cell walls in order to 

 obtain the required organisation for the primitive Metazoa." 



One difficulty comes when we consider whether the syncytium 

 has any inner cell walls or not. Thus if it can have cell walls 

 which do not divide the parts completely, then animals such as 

 Volvox which has connexions between the adjacent cells are 

 syncytia (Fig. 10). It is true that it is possible to consider that 

 Volvox has arisen by the accumulation of Chlamydomonas like 



