INTRODUCTION 21 



Bars of denser ehitinous substance form an internal skeleton; 

 special retractile fibers draw in a protrusible proboscis; similar 

 fibers closing a dorsal and a ventral operculum; other fibrils, func- 

 tioning as do nerves of Metazoa, form a complicated coordinating 

 system; cell mouth, cell anus and a fixed contractile vesicle or 

 excreting organ are also present. All of these are differentiated 

 parts of one cell for the performance of specific functions, and all 

 perform their functions for the good of the one-celled organism which 

 measures less than -jto mcn m length. Analogous, if not so com- 

 plete intracellular differentiations are present in the majority of 

 Infusoria, while many of the flagellates, notably the Hypermastigida, 

 have an almost equally elaborate make-up. In all such cases the 

 single cell is a complicated mechanism and the cooperating parts 

 have the same relation to the organism as a whole as do the organs 

 of a metazoon. Compared with an Amoeba proteus or other simple 

 rhizopod such complex organisms are highly specialized and show 

 the extent to which intracellular differentiation may be carried. As 

 Gurwitsch, Hartmann, Dobell and others have pointed out, the 

 application of the term cell which designates a structural unit with 

 specific physiological activity in Metazoa seems to be inappropriate, 

 and, as Whitman argued, inadequate. 



A significant difference between Protozoa and Metazoa lies in 

 the phenomenon of reversibility. Differentiations in the protozoan 

 organism are reversible and the derived organization is restored to 

 the fundamental organization (see p. 83) at periods of division, 

 parthenogenesis and fertilization. This does not occur in Metazoa 

 where differentiated cells derived from the fundamental organiza- 

 tion of the egg are irreversible and the "somatic" individual dies. 



Cell aggregates or colonies are likewise highly variable in their 

 functional specialization. AYhile many of them consist of fortuitous 

 groups of cells with dimensions varying with the number of indi- 

 viduals joined together {e.g., Ophrydium versatile, Poteriodendron 

 petiolatum, etc.), others are definite in form, number of cells and 

 in arrangement. Here the colony as such has a distinct individ- 

 uality and in some cases (e. g., Zoothamnium alternans) under- 

 goes a definite developmental cycle. Again some colonies com- 

 posed of otherwise independent cells do not react as separate 

 individuals but the colony reacts as a coordinated whole. Thus 

 Zoothamnium arbuscula, composed of many hundreds of individual 

 cells in a colony which may attain a diameter of 1 inch, reacts 

 as a unit organism if any one of the component cells is irritated. 

 The entire aggregate contracts into a small ball, so minute that 

 it is scarcely visible. The concerted action is due to the con- 

 traction of stalk myonemes which are continuous throughout the 

 entire aggregate, like the coenosarc of some hydroid colonies. 



