158 SPONGES 



feature of the forms mentioned is the relatively very large size of 

 their spicules, which is probably not a primitive characteristic. 



In general the Palaeontological record shows the extreme 

 antiquity of the chief types of sponges, and their wide occurrence 

 at all periods of the world's history, but is far from supplying links 

 to connect the branches of the phylogenetic tree. We notice 

 further that the forms most abundant as fossils are those which are 

 now characteristic of deep water ; but it is not necessarily to be 

 inferred from this that the fossil forms were also inhabitants of the 

 abyss. 



VI. THE AFFINITIES AND PHYLOGENY OF SPONGES. 



The theoretical questions which are suggested by a study of the 

 group of sponges, fall naturally under two headings. First, we 

 may consider sponges in general in their relation to other classes 

 of living beings. We are then confronted with the question 

 what are the affinities of the simple and primitive sponge individual 

 with other animals ? Secondly we have to deal with those questions 

 of which the range is limited and restricted within the phylum 

 itself that is to say, the evolution of sponges in general, and the 

 pedigree and phylogenetic relations of the principal groups. 



(a) Position of Sponges in the Animal Kingdom. The most con- 

 flicting opinions have been, and still are, held upon this point. Up 

 to the middle of the present century it was still disputed whether 

 sponges were animals or plants. The discovery of cilia in them by 

 Dujardin and Dobie was considered a decisive proof of their animal 

 nature, but their systematic position still remained a matter of 

 controversy. By Dujardin, Lieberkiihn, Carter, James-Clark, and 

 Savile-Kent, they were regarded as Protozoa, but with the progress 

 of knowledge such a view has become incompatible with any 

 rational definition that could be framed to separate the Protozoa 

 from the higher animals. Modern authors are divided, in the first 

 place, as to whether the sponges are to be regarded as Enterozoa, 

 or as an independent phylum, distinct both from the Protozoa and 

 from the Enterozoa (Biitschli, Sollas, and formerly Delage [2]). 

 Those who regard them as Enterozoa are further divided in opinion, 

 especially as regards the homologies of the two primary germ 

 layers. Balfour, whom at the present time Maas and Delage 

 follow, considered them as composed of ectoderm and endoderrn, 

 homologous with the similarly named layers in Coelentera, but in 

 sponges reversed in position at the metamorphosis ; Heider, Gotte, 

 and Noldeke also consider that sponges have nothing in common 

 above the gastrula stage, and the two latter believe that the sponge 

 body is developed from the endoderm alone. On the other hand, 

 Leuckart and Haeckel regard sponges as true Coelentera, composed 

 of the same two primary germ layers, and built up on the same 



