CHAPTER 5 



ORIGIN OF THE METAZOA 



When the basic assumptions underlying evolution were dis- 

 cussed on p. 13 it was pointed out that if the modern living forms 

 were polyphyletic, it should prove difficult to decide their inter- 

 relationships and we should have a number of isolated groups of 

 animals. This is precisely what we have discovered so far. The 

 Viruses, Rickettsiae, Bacteria and Protozoa are all quite distinct 

 from one another and their interrelationship is anything but 

 clear and certain. We come now to the Metazoa and we have to 

 decide whether they can be linked to any of the lower groups of 

 animals. 



There are three main views concerning the origin of the 

 Metazoa. These are that the Metazoa arose from (1) the colonial 

 protozoans, (2) the syncytial protozoans, and (3) the Metaphyta. 



Let us consider each of these views in turn. 



(i) Origin from colonial Protozoa 



Though the Protozoa are often defined as unicellular animals 

 there are many protozoans which after division or budding do not 

 separate their progeny so that the adult develops a colonial or 

 multicellular form. This development into colonies has taken place 

 many times within the Protozoa, as can be seen by looking at the 

 various classes. The most common examples are found in the 

 Flagellata. Simple unicellular forms such as Chlamydomonas can 

 at times exhibit an aggregated stage. An example of this is the 

 palmella stage during which Chlamydomonas encysts and divides 

 asexually ; the results enclosed in a gelatinous case may be regarded 

 as a colonial form (Fig. 8). 



In Gonium sociale a group of sixteen Chlamydomonas like 

 individuals are associated together in a plate. All these forms are 



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