114 THE EVOLUTION OF THE META20A 



no such cells can any longer be found. Thus we see that this 

 intermediate layer represents a property which has been deve- 

 loped in Cnidaria partly progressively and partly regressively. 

 In this connection it is interesting to observe that these me- 

 sohyl cells — whether they are cnidocytes, or genital cells, or 

 J-cells — do not heed the imaginary limit betv^een the two epi- 

 thelia. We can observe how these cells freely pass this "limit" 

 in their amoeboid movements (Hadzi, 1907, 1911). I have 

 been able to show this both during the formation of buds 

 in Hydra (Had^i, 1909 a, b) and in other Hydrozoa (the medusa 

 buds at the manubrium of Hydromedusae, Hadzi, 1912 d), 

 as well as during the widely practiced migrations of cnidocytes 

 (Hadzi, 1907 a), and of the primitive sex cells, as had been 

 formerly shown by Weisman (1883). 



The Morphological Proof of the New Interpretation 

 of the Phylogeny of Cnidaria 



Muscular Tissue 



Let us study now the muscles of Cnidaria. We must attribute 

 these to the middle layer even in those cases where the con- 

 tractile fibres do not form an independent middle layer, so that 

 it appears only as an appendix either of the ectodermal or of 

 the entodermal epithelia. Such a situation is generally consi- 

 dered as a very primitive one. We wish to show here that the 

 same is true for the muscles as has been said for the whole 

 middle layes, i.e. their development has been progressive or 

 regressive, concordant with the changes of the type of move- 

 ment. The consequence of the sessile way of life has been a 

 simplification of the cutaneous muscle tube, and, parallel to 

 this, a specialization of the body muscles in the movements 

 on a given spot. A new specialization has taken place during 

 the secondary transition to the free movement, which applies 

 especially to the medusoid form, yet this specialization again 



