Relationships of the Porifera. 253 



whether what follows is purely hypothetical, and how far it is 

 so. What is the ancestral form from which the Sponges have 

 been derived? This question has been answered in a variety 

 of ways ; but all the answers are hypothetical, for our embryo- 

 logical knowledge is too limited and imperfect. It appears to 

 me that it is as yet simply impossible to say what may have 

 been the appearance of the ancestral sponge. We have, it is 

 true, reason to believe in the existence of a free-swimming 

 form, which may have looked something like the larva of a 

 siliceous sponge, but not like that of Sycandra or similar 

 certainly aberrant forms. 



Before Leuckart's time (1854) the Sponges were regarded 

 as undoubted Protozoa. But when their complex structure 

 gradually became known, and especially after Huxley's state- 

 ments concerning the presence of ova and spermatozoa in 

 Tethya^ Leuckart first expressed the opinion that the Sponges 

 belong to the Coelenterata j and, indeed, up to a short time ago 

 this was the generally accepted hypothesis ; until at length 

 the third possibility was perceived, namely that they might 

 occupy a separate position between the two. This view has 

 again, found an advocate in Heider's latest work. In 1880 

 I indicated it in my Inaugural Dissertation. Balfour * is 

 of opinion that they form an " independent stock " of 

 Metazoa, and Sollas also. There can scarcely be any doubt 

 that the Sponges are not Protozoa. It is also certain that 

 there are, on the other hand, important differences between 

 true Coelenterata and Sponges. Even those investigators who 

 enthusiastically maintain the Coelenterate nature of the Pori- 

 fera place them as a natural, separate group, in opposition to 

 the Cnidaria. We are not, however, dealing only with the 

 question. Are the Porifera a subtype of the Coelenterata or 

 a special type? but also with the phylogenetic reasons. 

 Although the Sponges may not be Protozoa, yet they may 

 have descended from Protozoa. If we can hold in general 

 that the Metazoa are descended from Protozoa, and if we 

 further admit that Sponges are true Metazoa, then forth- 

 with we stand face to face with the question, What are the 

 phylogenetic relations of the Sponges to the remaining Meta- 

 zoa ? With regard to this the results arrived at by Sollas 

 and Biitschli essentially agree. Biitschli maintains " that the 

 Sponges form a group which is completely separated from the 

 remaining Metazoa and which originated from the Clioano- 

 flagellata (Saville Kent) quite independently." Indepen- 

 dently of Biitschli, Sollas came to the same conclusion; he 



* ' Comparative Embryology/ i. p. 122. 



