THE POSITION OF SPONGES, ETC. 435 



intimate relationship of the two groups, as they nozu exist is 

 based rather on affinities of analogy than homology." In 

 reply to this admission, which drew from Haeckel the retort 

 that Kent did not so much as know the difference between 

 homology and analogy, Lankester (1870) pointed out that 

 if the structures common to the two groups be derived by 

 descent from a common ancestor they are homologous. To 

 this Kent made the somewhat weak rejoinder (1870, p. 251) 

 that the admission of Haeckel's Protascus was " meant to 

 carry with it the vaguest possible significance and simply as 

 an acknowledgment of my faith in the doctrine of evolution," 

 and states his belief that " the open sac-like contour of all 

 existing coelenterates and a few exceptional spongiadse has 

 probably been arrived at by two very different processes of 

 evolution". Kent further sought to contrast the manner in 

 which sponges secure food by ciliary action, as "a purely 

 mechanical and involuntary force " with the voluntary mus- 

 cular force exerted by an Actinian in seizing its prey (p. 

 208), a comparison which, as Lankester remarked, proposes 

 to distinguish the two groups " by psychical manifestations" 

 (p. 87). Lankester points out that in Kent's manner 

 of "viewing" sponges all organisms might be classed as 

 Protozoa, and concludes that the result of Haeckel's argu- 

 ments will be to cause naturalists to admit the separation of 

 sponges from Protozoa, but that " he will have greater diffi- 

 culty in getting the association with Coelenterata, and with 

 Anthozoa in particular, admitted ". 



In 1872 Eimer published a paper describing thread 

 cells in sponges. Carter (1872) at once pointed out, how- 

 ever, that these thread cells did not belong to the sponge 

 but to symbiotic hydroids living in the sponge tissue, and 

 nematocysts have not since then figured in sponge literature. 

 Haeckel, however, in his monograph of the calcareous 

 sponges, published in the same year, modifies his view of 

 sponge affinities. It is now the hydroids and not the 

 corals with which he tries to compare sponges. He re- 

 gards the most primitive sponges as related to the most 

 primitive Acalephse, but " Sponges and Acalepha; are two 

 diverging branches of the Zoophyte stem which have 



