ORIGIN OF THE METAZOA 47 



individuals whilst it is not possible to find any sub-unit for 

 Opali?ia. Nevertheless the differences between syncytia and 

 colonies are not as clear cut as has sometimes been supposed. 



Another difficulty arises when we consider the stage during the 

 life cycle during which the protozoan is syncytial or colonial. 

 Thus many of the Protozoa form spores during reproduction and 

 these may be localised in spore cases. Are these to be regarded as a 

 multicellular stage? If so, then the palmella stage of Chlamy- 

 domonas could be a colonial form (Fig. 8). The Cnidosporidia 

 have many nuclei only during the reproductive (spore-forming) 

 stage; their trophozoite is unicellular and only has one nucleus. 

 Equally well the ciliate Anoplophrya, which does not separate 

 its asexually produced cells from the parent immediately they are 

 produced, can be considered as a colonial form (Fig. 14). In 

 effect the situation is quite difficult to resolve and depends to a 

 large extent on the relative duration of the multicellular stage and 

 the part that it plays in the life of the animal. Hadzi (1953) 

 has suggested that one of the major differences between the 

 Protozoa and the Metazoa is that the Protozoa have their major 

 phase in the reproductive stage whilst the Metazoa have their 

 major phase in the vegetative stage. 



(3) Origin from the Metaphyta 



The third view concerning the origin of the Metazoa is that 

 they arose from the plants, the Metaphyta. It has already been 

 mentioned that Franz (1926) thought that the Protozoa were in 

 fact derived from the Metaphyta and the Metazoa. Baker (1948) 

 suggested that the Metazoa arose from plant-like protozoans. 

 " The unicellular plant absorbs nutriment from all sides equally, 

 and when in the course of ontogeny or phylogeny it becomes a 

 metaphyte there is no fundamental change in this respect; a cell 

 divides without separation and the two products continue to 

 absorb nutriment over most of their surface. The passage from 

 unicellular form to the metaphyte is therefore easy. In the case of 

 animals, however, there is an important change when a unicellular 

 form becomes a metazoon; a new method of feeding must be 

 adopted. . . . The difficulties would be greatest when the pro- 

 tozoon had a localised mouth. If the products of such an animal 

 were to adhere together and each were to acquire its own mouth, 



