6 THE EVOLUTION OF THE METAZOA 



and is reproduced in the present volume (Fig. 1). According 

 to this hypothesis the evolution of Spongiae progressed com- 

 pletely independent from that of Eumetazoa. The starting 

 point was probably the freely swimming colonies of Mastigo- 

 phora of the subtype Choanofiagellata. The subindividuals were 



Fig. 1. Hypothetical construction of the evolution of the sponges. 

 A, free swimming phase resembling Proterospongia sp.; B — C, the 

 beginning of the sessile phase; D — E, erect phase with the for- 

 mation of chambers; F, primitive phase of the heterocoelic state; 

 G, homocoelic phase. (After Hadzi, somewhat altered.) 



kept together by means of a richly secreted jelly in a way 

 which can even now be observed among Choanofiagellata in 

 the genus Proterospongia. The existence of this form of Choano- 

 fiagellata, which was first described by Saville Kent, was 

 disbelieved and it was considered that we did not have here an 

 independent animal but rather a surviving fragment of a real 

 sponge. In the meantime, another species of the same genus 

 has been described, so that now we have not only the old s^e- 

 d.QsProterospongia baeckelii S. Kent, (Grontved, 1956; Lackey, 



