48 ORIGIN OF THE METAZOA 



no advance could be made to the evolution of a metazoan alimen- 

 tary canal. This suggests that the Metazoa may have arisen 

 from primitive Protozoa unprovided with localised organs of 

 assimilation." 



Hardy (1953), following on from Baker's argument, suggests 

 that " the Metazoa have not been derived from the Protozoa at all 

 but from relatively simple metaphytes, which after they had 

 evolved from the protophytes began, perhaps as a result of a 

 shortage of phosphates or nitrates, to capture and feed on small 

 organisms as do the higher insectivorous plants." 



This is an interesting suggestion but one that can be criticised 

 on several grounds. There is no evidence that metaphytes such as 

 the Algae can withstand food shortages by catching animacules. 

 The thick cellulose cell wall around the metaphytes, though a 

 protection, would also tend to prevent them from developing 

 pse dopodia rapidly enough to catch protozoans. The insectivor- 

 ous plants, it will be remembered, all develop special insect- 

 catching mechanisms, and even so they still retain their photo- 

 synthetic ability. 



Baker's arguments in favour of the origin of Metazoa from 

 plant-like protozoans can also be contested. There is no reason 

 why a metazoan type of alimentary canal should develop in the 

 first stages of the evolution of the Metazoa. The " alimentary 

 canal " of the sponges is not really comparable in function to that 

 of the higher metazoans, and even in the coelenterates there is a 

 considerable amount of amoeboid activity in the gut cavity. What 

 would appear to be more important than the development of an 

 alimentary canal is that the cells of the body should have some 

 continuity and interconnexion with each other so that food 

 material can be passed easily, from one cell to the other. Such a 

 process probably does occur in the colonial ciliates such as 

 Zoothamnion which have well-developed gullets. Summers (1938) 

 suggested that food material was probably passed along the stalk 

 of Zoothamnion. Here then we have the case of a protozoan with a 

 well-defined mouth forming a colony. Furthermore this colony 

 shows differentiation and division of labour, some of the polyps 

 being more intensive feeders than others. 



There is no reason why a protozoan with a definite polarity 

 should not lose this polarity and develop into a colonial form. 



