General Morphology of the Protozoa 3 



phidae (Fig. 1. 10, D). In addition to normally multinucleate Protozoa, 

 many species are uninucleate in one phase of the life-cycle and multi- 

 nucleate in another. 



Such structural diversity has led protozoologists into difficulties with 

 the Cell Theory. Dobell (45), who suggested that Protozoa are non-cel- 

 lular organisms, was one of the first to revolt against strict application of 

 the Cell Theory to this group. Such an interpretation has appealed to 

 some zoologists. A different concept, favored by Kofoid (138) for example, 

 is that some Protozoa are unicellular while others are multicellular. 

 Protozoan "multicellularity" is considered analogous to metazoan multi- 

 cellularity as seen in syncytial tissues. According to this view, the Protozoa 

 are the phylum in which multicellularity originated in animals. 



The evolutionary transition from Protozoa to Metazoa involved dif- 

 ferentiation beyond the separation of reproductive and somatic cells. 

 Hyman (98) has stressed the characteristic establishment of an axis along 

 which morphological and physiological differentiation has occurred. Such 

 colonial types as Volvox, in spite of their specialized somatic and repro- 

 ductive "cells," are usually considered Protozoa. The distinction is mainly 

 one of degree, since Volvox has several attributes of an organism in the 

 metazoan sense. The colony moves as a unit, with apparently coordinated 

 flagellar activity, and exhibits some degree of polarity with functional 

 differentiation. The colony may produce daughter colonies asexually or it 

 may develop gametes. The zygote develops into a young colony in a man- 

 ner not unlike that in which a fertilized egg produces a young metazoan 

 individual. The Myxosporida, another exceptional group, show somatic 

 differentiation in that some cells produce spore-membranes while others 

 give rise to the polar capsules of the myxosporidian spore. In other words, 

 the separation of Protozoa from Metazoa in borderline cases may involve 

 somewhat arbitrary decisions influenced to some extent by factors of 

 taxonomic convenience. 



VARIATIONS IN FORM OF 

 THE BODY 



Protozoa range from approximately spherical forms to bizarre 

 shapes not readily explained on a functional basis. Symmetry is often 

 poorly defined. Most active swimmers show spiral torsion in some degree 

 and this tendency toward asymmetry is presumably correlated with the 

 usual spiral course in locomotion (62, 136). However, universal sym- 

 metry and radial symmetry may be noted in various floating and sessile 

 species, respectively, and bilateral symmetry is apparent in such genera 

 as Giardia and Octomitus. In Protozoa which are not spherical, form of 

 the body may be rather characteristic of a given species under particular 

 conditions. However, form is often relatively constant rather than abso- 

 lutely so, and within specific limits, may be modified by environmental 



