CILIARY SYSTEM, ONTOGENY AND PHYLOGENY 71 



anteroposterior band. The phenomena, if not identical 

 with, are very similar to, those taking place in Proboveria. 



Ancistruvi, Proboveria, and Boveria are obviously closely 

 related forms from the biological, morphological, cytologi- 

 cal, and developmental points of view. The position of the 

 origin of the preoral system is the main difference. As a 

 matter of fact, this position varies somewhat in some species 

 of Ancistrum. 



It is difficult to escape the feeling that Hetnispeiridae 

 represents a monophyletic group and that Proboveria and 

 Boveria have originated from a common ancestor of the 

 Ancistrum type, w^hich is the primitive structural form of 

 the family. 



Let us assume that the position of the oral system is the 

 consequence of "properties" of the cortex. We have to ad- 

 mit that, before division, constitution and structure of the 

 cortex, as judged by its effects on the oral system, are equiv- 

 alent to those of the primitive type. 



The evolutionary series — Ancistrum -^Proboveria -^ Bo- 

 veria — consists in an anteroposterior migration of the oral 

 system which becomes enrolled on the posterior end. This 

 migration is reversed before division. The predivisional 

 type of structure corresponds to the phylogenetically primi- 

 tive state; the adult's structure, by definition, to the evolved 

 state. The movements of the oral system therefore appear 

 as the consequence of reversible changes of cytoplasmic 

 properties. 



Evolution is generally considered to be irreversible. 

 What is irreversible in the ciliates we are considering is the 

 position of the preoral cilia in the mature ciliate, which is 

 the result of ontogeny as modified by evolution. But before 

 division, the preoral system moves towards the anterior 

 pole. This movement is reversible. What is irreversible is 

 the structure considered at a given phase of the life cycle. 

 The consideration of apostomatous ciliates leads to the same 

 conclusion. 



