SPONGES 159 



architectural plan ; and Schulze, whom most authors follow, places 

 the Porifera as a subdivision of the Coelentera, marked off by the 

 possession of a distinct mesoderm, and the absence of nematocysts 

 or tentacles from the remainder of the Coelentera or Cnidaria. 



We have then three views to discuss, each of which is based 

 upon a distinct theory of sponge genealogy : first, that sponges are 

 descended from a Protozoon ancestor distinct from that of other 

 Metazoa ; secondly, that they have a common ancestry with other 

 Metazoa, as far as the diploblastic stage of development, and are 

 therefore composed of the same two primary germ layers, but that 

 after this stage they have proceeded along a distinct and in- 

 dependent line of evolution ; and thirdly, that they have a common 

 ancestry with the Coelentera, both being descended from a 

 gastrula-like progenitor with a body wall composed of ectoderm 

 externally and endoderm internally. The sponges would then be 

 a modification of the Coelenterate ancestor in one direction, the 

 Cnidaria in another. 



In considering these views we may take the last first, as being 

 the most easy to refute, although still that most generally adopted. 

 The result of recent embryological work has been to completely 

 undermine the Coelenterate theory of sponge affinities, by demon- 

 strating that at the metamorphosis the germinal layers become 

 reversed in position, in a manner not suspected by those who 

 first put forward this view. The Coelenterate theory assumes 

 that sponges are composed developmentally of the same two 

 germinal layers as the Coelentera, which also have the same archi- 

 tectural relations in the adult. The reversal of the layers makes it 

 impossible, however, to extend the comparison to both the larvae 

 and the adults. For if the comparison starts from the larvae, then 

 the adult sponges are composed of endoderm externally and ecto- 

 derm internally. If, on the other hand, the adults are compared, 

 and their constituent layers homologised, then the larvae of sponges 

 are quite anomalous, consisting of an endoderm surrounding com- 

 pletely, or very largely, the ectoderm. Since the Coelenterate 

 theory has become quite untenable at the present day, at least in a 

 strict phylogenetic sense, we have to choose between one of the two 

 remaining views : either that sponges have a separate descent from 

 the Protozoa, i.e. from the Choanoflagellata, an idea which has sug- 

 gested the term Parazoa, applied to them by Sollas ; or that they 

 are Enterozoa, in which the two primary germ layers have become 

 reversed in position, a view expressed in the name Enantioderma s. 

 Enantiozoa 1 coined for them by Delage (18D8 [3]). 2 



1 From the Greek fvavnos, inside out. 



2 The theory of Gotte arid Noldeke that sponges are developed from the endo- 

 derm alone, was founded on mistaken observations upon the development of 

 Spongilla, and at the present day is lacking in any basis of fact. 



