PHYLOGENY 



to the ciliates. Adult suctorians have no ciha, nor have they any other 

 locomotor organelles. Nor do they have a mouth, for they use protoplasmic 

 tentacles for the capture and ingestion of food. They are generally per- 

 manently attached to their substrate. Up to this point, they show no 

 affinity with the ciliates. However, they do have the two types of nuclei 

 and sexual reproduction by means of conjugation, phenomena which are 

 unknown elsewhere in the world of life except for these two classes. Fur- 

 ther, the suctorian zygote becomes a free-swimming, ciliated organism 

 which only later settles down on the substrate and adopts the typical suc- 

 torian mode of life. This is interpreted as embryological recapitulation of 

 ancestral history. 



ORIGIN OF THE METAZOA 



Hyman has said that there is no direct proof of the origin of the Metazoa 

 from the Protozoa, yet the discussion of the origin of the Metazoa ( multi- 

 cellular animals ) usually revolves around the question of which protozoan 

 stock would seem to be the most probable progenitor of the Metazoa. Two 

 broad possibilities exist by which the Metazoa could have been formed 

 from the Protozoa. The first is that repeated nuclear division without 

 cytoplasmic division might have led to formation of a plasmodium, like 

 some of the Heliozoa. Formation of cell membranes would then result in 

 multicellularity, and differentiation might then lead to the true multi- 

 cellular individual. The second method is the differentiation of cells within 

 a colony of Protozoa, comparable to Volvox, leading to interdependence 

 and individuality. 



A very different, third possibility has been suggested. In protozoan 

 colonies each cell ingests food, but even in simple metazoans, a new 

 method of feeding is used, with a digestive tract feeding for the whole 

 organism. This transition might be difficult. Hardy suggests that simple 

 plants, like Volvox, living in an environment deficient in nitrates and 

 phosphates, may have satisfied the deficiency by capturing smaller organ- 

 isms. Increasing utilization of this nutritive pathway, together with loss 

 of photosynthesis, would then lead to a simple metazoan. Insectivorous 

 plants show the feasibility of such a nutritive mechanism, and the fact that 

 unicellular plants seem to have given rise to protozoans more than once 

 lends plausibility to the suggestion that multicellular plants may have 

 achieved animalization at least once. 



Paleontology is of no help in this problem, for the Metazoa were already 

 well established at the beginning of the Cambrian. It is therefore probable 

 that the origin of the Metazoa will always be speculative. But most zoolo- 

 gists favor the flagellates as the most probable progenitors of the Metazoa. 

 The reasons for this are many. The flagellates are a highly variable group 

 which appear to have given rise to many other groups of plants and to 

 several, perhaps all, other groups of Protozoa. Further, some groups of 

 flagellates show a tendency to form colonies of ever-increasing size and 

 complexity. The evolution of sex has occurred here, and the colonies are 

 definitely divided into somatic and germinal "tissues." Oogamous repro- 



132 



