CNIDARIA AS THE ONLY COELENTERATA / 



1959). In this way the existence of freely living colonies of Cho- 

 anoflagellata can no longer be doubted. Of course, this can only 

 be an example in our attempt to reconstruct the phylogeny of 

 Spongiae. When an organism which was similar to Proterospongia 

 changed to a sessile way of life, the duty to stick to the floor of 

 the sea was taken over by a small number of basal choanocytes 

 losing their character as choanocytes. Later, some of the surface 

 cells that could stretch out freely into water took over the form 

 offlattenedamoebocytes and the latter developed the peculiari- 

 ties of pinacocytes. Depressions developed in several parts of 

 the surface when groups of choanocytes sank deeper into the 

 jelly. It was probably in this way that several chambers with 

 pores developed on the surface; the remaining surface was 

 covered with pinacocytes. The pore of the highest placed cham- 

 ber developed into an osculum when the internal connection 

 of chambers was evolved. The water flov.' through individual 

 chambers was united into a common draining system. The 

 amoebocytes in the jelly developed partly into the skeleton 

 cells and partly into the primitive sex cells. 



As a result of such an interpretation we find that the homo- 

 coelous type of Spongiae, the Ascon or Olynthus according 

 to Haeckel, cannot be primordial but rather a result of a secon- 

 dary development due to a secondary union of chambers. This 

 makes it clear that ascon cannot be a gastraea. The freely living 

 larva, the spongula, which is characteristic for Spongiae and 

 which actually has a very manifold structure, developed inde- 

 pendently and thus it cannot be considered to be a recapitu- 

 lated form. There were no primitive Spongiae whose form 

 and structure resembled those of the spongulae. The adoption 

 of a freely living, usually planktonic juvenile stage of Spongiae, 

 and, herewith, their prolonged ontogeny, corresponds to a 

 general phenomenon and is a consequence of the transition 

 to the sessile way of life by the adult form. The sexual stage 

 together A^ith the development (cleavage, etc.) which follows 

 fertilization cannot be considered as a proof of the common 

 derivation of all Metazoa from the colonies of Flagellata. The 

 2* 



