no 



THE EVOLUTION OF THE METAZOA 



Scyphozoa. In Hydroidea the polymorphous cormi usually ap- 

 pear with one medusoid form only and they are mostly irregu- 

 larly built ; the only regular form is that of a plume (Pennaria) 

 among Athecata and in the numerous species of Plumulariidae 

 among the Thecata (Fig. 20.). 



There are many other properties and characteristics of Cni- 

 daria which can be understood only if we interpret their phy- 



B 



Fig. 20. Polymorphic hydroidpolyps organized as a subcormus 



or cormidium, composed of a gastropolyp and a few heterozooids 



(machozooids). A, Aglaophem'a sp. (after Kiihn). B, Halicornaria 



cornnta Allman (after Allman). 



logeny as having followed the direction Anthozoa — Scypho- 

 zoa — Hydrozoa. In our analysis of these properties we must 

 be constantly aware of the fact that two parallel processes are 

 taking place in connection with the unique speciahzation which 

 the sessile way of life entails. One process is that of regression, 

 the retrograde development which has also been unsuitably 

 called, as has already been mentioned, degeneration (cf. 

 Laubenfels [1955], and the reply by M. Zalokar [1955]).. During 

 such a process all characteristics and peculiarities which 

 serve the purpose of a free movement are gradually lost. 



