43G THE DIGESTION OF SPONGES, 



This conclusion was somewhat similar to one which Balfour (1) 

 had arrived at by reasoning only and without any actual proof. 



If the Epithel of the introductory Canals is Entodermal, this 

 conclusion is wrong, because in that case one of the premises is 

 wrong. And further investigations will be recpiired to settle the 

 question of the correctness of these premises. 



I have made up my mind to publish this explanation, because 

 some authors have considered my conclusions as important, and 

 that part of the premises which is the result of my experiments as 

 of lesser importance. Polejaeff (2) has pointed out this fact in his 

 recent excellent publication on the Calcareous Spongea of the 

 Challenger Expedition. 



Metschnikoff (3) in his highly important paper on Intracellular 

 Digestion, has cited my observations as examples of Ectodermal 

 digestion, but it appears from his statements, as if I had said that 

 the outer svorface of the Sponge absorbed noimshinent, which I did 

 not say. 



Polejaeff (I.e.), remarks with great acuteness, that I should not 

 have considered the " Keimblatter " in my argument, as it is not 

 certain to which embryonic layers that Epithel belongs, by which in 

 the case of the Aplysillidas the greater part of its nourishment 

 appears to be obsejved. 



Polejaeff says in another place (4), that nourishment is absorbed 

 by all the Canal surfaces, that the Epithelia of the Canals conducting 

 hither and thither, are physiologically as similar as they are morpho- 

 logically. This opinion is based upon four arguments. Two of these 

 do not apply to the Aplysillidae. Polejaeff states that the amoeboid 



(1.) T. Balfour. Handbuch dcr vergleichenden Embryologie, deutsch von 

 Vetter. Band, I. Seite, 144. 



(±) Polejaeff. The Zoology of the Voyage of H. M. S. Challenger. 

 Part XXIV., p. 14. 



(3.) K. Metschnikoff. Untersuchungen iiber die intracelluliire Verdauung 

 bei w irbellosen Thieren. Arbeiten aus den Zoolog. Inst, der Universitoet 

 Wien. Band, V. Seite, 3. Translated into English. Quarterly Journal 

 of mikr., Sc, Nr. 98. 



(4.) Polejaeff. (La), p. lot 



