142 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



protozoa, the different species find a ready and reasonable 

 explanation for their origin in Symbionticism. The ciUate 

 and flagellate protozoa, apparently, acquired their special 

 locomotor structures through symbiosis with ciliate and 

 flagellate bacteria. We have previously mentioned that 

 some investigators have claimed that ciha formation in 

 the cells of higher organisms is associated with mitochon- 

 dria. So also the origin of those protozoa having the ability 

 to produce calcareous shells about their bodies, finds rational 

 explanation in Sjonbionticism. Bacteria with the property 

 of secreting calcareous materials are known. 



One of the simplest metazoa (many-celled animals) is 

 Pandorina, in which all the cells look ahke. It consists 

 merely of a colony of cells that do not separate, but remain 

 attached. On the basis of Symbionticism, it is readily 

 conceivable how such an organism may have arisen. It is 

 rational to suppose that a one-celled animal acquired a 

 microsymbiont of such properties that it modified the 

 physical nature of the host cytoplasm, which was so altered 

 that the cells after division remained together. This 

 cohesive property of the protoplasm was a fundamental 

 acquisition, for it has persisted in the development of all 

 higher animals. Obviously, the extent to which this phys- 

 ical property (viscosity) is found, varies in different groups 

 of cells. 



There are some animals that are considered "regressive" 

 organisms. One of the most widely-known groups of this 

 character is the Tunicata. The adult organism exhibits a 

 morphology of a lower order than the developmental or 

 ontogenetic history would indicate. These organisms have 

 aroused considerable interest among students of evolution, 

 and various hypotheses have been advanced to explain this 

 "retrograde" development. Presumably, the origin of 

 these forms is attributable to the same set of factors re- 

 sponsible for the production of the progressive types. It 



