436 SCIENCE PROGRESS. 



developed from the common ancestral stem of the Protascus" 

 (1872, vol. i., p. 461). In other words, it is no longer suggested 

 that sponges are a group of the same systematic value as one 

 of the divisions of the coelenterates, but that they are equiva- 

 lent in the system to all remaining classes of coelenterates 

 put together. Since Eimer had found nematocysts in 

 sponges their possession is no longer to be regarded as 

 characteristic of coelenterates, but a distinctive feature of 

 the two groups is the formation of pores in sponges, since 

 he now admits that the cutaneous pores of acalephs are 

 analogous and not homologous with those of sponges ; as 

 Kent had already declared to be the case. 



Finally, in 1889 (p. 90), Haeckel came to regard the 

 flagellated chamber as the primitive individual in sponges, 

 " homologous with a Gastraea on the one hand and with a 

 simple Hydroid on the other," and concludes : " The 

 position of sponges in the animal kingdom as a separate 

 phylum of the Coelenterata . . . seems to be the most 

 natural ". 



1 he change of position with regard to the coelenterate 

 theory of sponges which we see in comparing Haeckel's 

 earlier and later writings, finds a parallel in the different 

 stand-points taken by the authors who also support this 

 theory. Marshall alone takes an extreme view similar to 

 that in Haeckel's earlier works, and considers (1885) sponges 

 true coelenterates descended from a protactinia with tentacles 

 and a radiate gastrovascular apparatus, which persists as 

 the canal system. All other advocates of the coelenterate 

 nature of sponges take rather the view which may be said 

 to dominate in the text-book and the lecture room, that the 

 Coelenterata are to be divided into two distinct and opposed 

 groups, the Porifera or sponges, and the Cnidaria or polyps 

 and medusae. Schulze, whose opinion carries more weight 

 than that of any other authority, after remarking on the simi- 

 larity of sponge and coelenterate larvae, points out that the 

 metamorphosis indicates the point at which a separation took 

 place in phylogeny, but finds it difficult to determine what 

 degree of organisation was reached before the separation took 

 place. He considers, however, that there is very little to sup- 



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