NEW INTERPRETATION OF CNIDARIA 327 



time they differ basically from all the other polycellular animal 

 forms (Eumetazoa). Not only had they evolved into their 

 polycellularforms from their protozoan ancestors along a com- 

 pletely different way, i.e. by way of some colonies of Flagellata, 

 they also never developed a true digestive cavity which could 

 be considered as a homologue of the cavity that occurs in the 

 Eumetazoa: their chambers that are surrounded by choano- 

 cytes are not digestive cavities, i.e. they are not a multiplied 

 intestine. The choanocytes are not gland cells, or intestinal 

 cells; they digest in their own organelles, in the digestive 

 vacuoles, in the same way as the solitary or colonial Choanofla- 

 gellata do. The Spongiae have neither an intestine nor an oral 

 orifice. The choanocytes which are actually similar to the 

 Choanoflagellata can only become amoeboid; as such they 

 had been somewhat differentiated, yet they have never been 

 able to develop a genuine tissue, and even less so an organ 

 or systems of organs. 



Under the influence of the old interpretation zoologists have 

 tried to explain the internalization of an originally external 

 digestive surface. It was supposed that the digestive surface 

 had been originally situated externally on the body of the ani- 

 mal because the first colonies (blastaeae) had consisted, accord- 

 ing to this interpretation, exclusively of individuals that gather- 

 ed their food independently, W' hich they digested each in their 

 own vacuoles. These zoologists think that it was only secon- 

 darily and after the first differentiation had taken place, that 

 the digestive surfaces w^ere transferred into the interior of the 

 animal body, w^hich was subsequently covered with the 

 skin cells. This transference into the interior was supposed 

 to have taken place in different w^ays as this is shown by the 

 patterns that can be observed in the ontogenies. The inva- 

 gination of the posterior half into the anterior half was 

 considered as the most primitive and typical w^ay of the develop- 

 ment of the intestine (the invagination gastrula). All this is 

 extremely improbable and it is not supported by facts. These are 

 pure reconstructions made on the basis of facts that can be 

 22* 



