120 THE EVOLUTION OF THE META20A 



the main function is taken by the transversally placed 

 layer of muscular fibres which adjoins the entodermal epi- 

 thelium. 



The muscular conditions that can be observed in Cnidaria 

 —not less than those that can be observed in Turbellaria— show 

 clearly that so far it has been much exaggerated when attempts 

 have been made to establish a sharp division between the skin 

 and the intestine on one hand which both show a strong in- 

 clination to the epithelization, and the mesohyl on the other 

 as the layer which is situated between them. We are justified 

 in maintaining, on the basis of known facts, that in Cnidaria 

 and in Turbellaria the cytoplasma has the ability to develop 

 special strongly contractile fibres (the muscle fibres), regardless 

 of w^hether it has become differentiated as skin cells (ecto- 

 derm), intestinal cells (entoderm), or mesenchyme cells (me- 

 soderm or mesohyl), as w-ell as the ability to grow^ undulipodia 

 (cilia or flagella) on its free surfaces (either external, or in- 

 ternal, or intermediate). This process is reversible, i.e. the abi- 

 lity can be lost again, and under special conditions it can again 

 become active. 



The mesenchymal muscle system w^hich plays a major role 

 in Turbellaria has been lost in Cnidaria, and the "mesenchyme" 

 itself has become strongly reduced. These muscles, how- 

 ever, have been preserved in Ctenophora, w^hile in Cnidaria 

 w^hich have adopted the sessile way of life they have become 

 more closely connected with the tw^o body epithelia and it has 

 been only secondarily that they have occasionally again liberat- 

 ed themselves from the epithelium cells and sunk under the 

 epithelium. 



It should be mentioned here that in some primitive Turbel- 

 laria (e.g. in the aloeocoelous species Prorhynchus baswel/i accord- 

 ing to Steinbock and Reisinger; Bresslau, 1933) the con- 

 tractile muscle fibres have been developed in the skin stratum 

 itself, i.e. in the cytoplasm of the skin epithelium, and thus 

 intraepithelially. This development has been so rich that here 

 they form even four strata, i.e. in the distal zone (in the "cover 



