48 M. E. Metschnikoff on the 



the agreement of the outer layer only the absence of cilia, a 

 negative and unimportant character, has been cited. But as 

 regards the differences, the different structure of the outer 

 layer must be placed in the first rank. Hackel endeavours to 

 get over this difficulty, saying : — " but this [difference in the 

 structure of the exoderm] is to be regarded as a secondary histo- 

 logical differentiation of the two divergent groups" (p. 460). 

 Although he cites no evidence of this, he has no hesitation in 

 explaining the u differences in anatomical structure between 

 the simplest hydroids and the simplest sponges" as u of quite 

 subordinate significance " (p. 460). But is it really so insigni- 

 ficant that the outer layer of the sponge exclusively produces 

 all skeletal formations, whilst in the true Cceleuterata these 

 are never developed from the ectoderm, but always from the 

 cutis (therefore from the mesoderm) ? Where do we know of 

 any examples of an epithelial tissue (to which the ecto- 

 derm of the Ccelenterata belongs) serving as the seat of the 

 formation of a calcareous skeleton ? These are questions for 

 an answer to which we may seek in vain from Hackel. 



Let us return to Hackel's argumentation : — At p. 461 we 

 read as follows : — " Of the greatest significance is the onto- 

 geny of Cordyloplwra, which perfectly agrees with that of 

 Olynthus." Unfortunately Hackel knows so little of the 

 ontogeny of Olynthus, that he has no right to say any thing 

 about this " perfect agreement." As we have seen, Hackel 

 has invented the metamorphosis of the Calcispongiaj (without 

 hitting upon the right thing), in doing which he evidently 

 took the agreement with the Hydroida as his starting-point, 

 instead of arriving at it as a result. In my opinion the meta- 

 morphosis " directly inferred " by Hackel is nothing more than 

 a cast (AbJclatsch) from the well-known metamorphic history 

 of the Hydroida. Hackel says with particular emphasis that 

 il the Planida and the Planogastnda are perfectly alike in both 

 animals;" but that proves nothing so long as neither the 

 origin nor the metamorphosis of the ciliated larva has been 

 observed*. 



Hackel may repeat, as often as he pleases, that he was the 

 first to demonstrate the homology of the two lamella? of the 

 Sponges and Ccelenterata f ; but every critically thinking natu- 

 ralist will at once see that this is not the case, and that in 



* The transformation of his " morula " into the swimming larva has 

 not been observed by Hackel any more than by myself; he has neither 

 described nor figured any transition-stage ; nevertheless he feels justified 

 in filling up the existing gap a priori, without, however, expressly 

 saying so. 



t See the above-cited quotations from p. 214, and, further, pp. 33, 456, 

 and 470. 



