PORIFERA 29 



cases the common fundament of the ampullae and exhalent 

 canal system is affiinned to have the form of a central 

 cavity (archenteron), fi'om which the ampullte and exhalent 

 canals arise by repeated foldings of the wall. Opposed to 

 this are the observations of other authors, according to 

 whom the different ampullae are formed independently, and 

 become united by means of the canals which appear later 

 on, and whicli only gradually unite into a common system 

 of canals. 



In such a condition of aifairs it is hardly possible to 

 arrive at any general conclusions without in one way or 

 another doing violence to the individual statements. This 

 much, however, seems with some certainty to result from all 

 the observations : that we have before us in the sponges an 

 independent stem of the Metazoa, which is connected with 

 the other types only at its roots. We adhere to the view 

 that the sponges have a common origin with the i^est of the 

 Metazoa. In the embryology of the sponges we find true 

 blastula- and gastrula-stages, which appear to point to 

 an ancestral form common to the Porifera and all other 

 Metazoa. Characteristics of histological differentiation (the 

 formation of columnar and pavement epithelium, of con- 

 nective tissue and cartilaginous tissue) likewise point to this 

 community of origin. As opposed to these characteristics, the 

 single fact of the occurrence of the collar-bearing flagellate 

 cells of the entoderm does not seem sufficient to wari^ant the 

 derivation of the Porifera as an independent group from the 

 Choanoflagellata and the denial of their phylogenetic 

 relationships to the rest of the Metazoa (Sollas, N'o. 15 ; 



BÜTSCHLl). 



That the Porifera do not stand in any close relationship 

 to the Cnidaria (Coelenteratain the stricter sense) (Marshall, 

 No. II) appears to be beyond doubt. We lay less stress upon 

 the absence of nettling capsules, as being a purely histological 

 character, than on tectonic points. The attempts to reduce 

 the structure of sponges to the fundamental form of the 

 Polyp must lead to contradictions. Above all must be 

 emphasized the fact that the exhalent opening of the canal 

 system, the so-called osculum, is not homologous with the 



