Development of the Calcispongise. 51 



logy can be accepted only for two embryos. The one-layered 

 external envelope is in all three cases the dermal layer, which 

 afterwards becomes covered with cilia and represents an epi- 

 thelial tissue, which may be 'characterized throughout as the 

 ectoderm. In the siliceous sponges this layer is only of short 

 persistence ; it disappears during the transformation into the 

 attached form. In the Echini also the ectoderm is only 

 provisional, at least upon many parts of the body. In the 

 Hydroida, on the contrary, it persists throughout life, as is 

 sufficiently well known. The inner cell-mass, in our three 

 cases, experiences the following alterations : in the siliceous 

 sponges it furnishes (at least for the most part) the skeleton- 

 forming layer, becoming converted into the so-called syncytium 

 of Hackel ; in the Echini it plays a perfectly similar part, 

 although the cellular elements here retain their individuality ; 

 but it is quite otherwise in Sertularia (and the Hydroida in 

 general), in which the cell-mass, although similar in appear- 

 ance, becomes the entoderm. 



The conclusion at which I have arrived is, that the syn- 

 cytium corresponds to the skeleton-forming layer of the Echi- 

 nodermata (and Ccelenterata) , whilst the ectoderm (in the 

 siliceous sponges) appears as a provisional structure confined to 

 the larval stage. (With regard to the inner layer (b) of our 

 three embryos, a still more profound analysis may be instituted; 

 we may elucidate the question as to the homology of this layer 

 by the consideration of the mode of origin of the mesoderm. 

 But this we shall pass over, so as not to depart from the prin- 

 cipal theme, especially as at the moment several important 

 facts are still insufficiently known.) 



What, then, is the position of the Calcispongire in relation 

 to the question of the germ-lamellse ? With regard to this 

 order in general nothing definite can be said at present, as the 

 larvae of different representatives of the order appear to be con- 

 structed in various ways, while the history of the metamor- 

 phosis is known only in the case of a single species. But if we 

 take this species alone into consideration, we may, by compa- 

 rison with the better-known siliceous sponges, obtain an un- 

 derstanding of many circumstances. Above all it must be 

 borne in mind that the larvae of four genera of marine siliceous 

 sponges observed by me always have a gap in the ectoderm at 

 the posterior end of the body through which the skeleton- 

 forming layer projects outwards. Now this baring, which, 

 indeed, is very peculiar, occurs in a still greater degree in the 

 Sycon-laYvse, which is in connexion with the weak development 

 of the ciliated layer. The latter, instead of forming a sphere 

 as in so many other animals, remains only in the form of the 



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