17 



falthough the full paper with the accompanying plates is now in proof) 

 I may perhaps be allowed to give a brief résumé of my arguments in 

 this place. 



In the genus Siphonochalina we meet with three species — S. spi- 

 culosa, S. procumhens and S. ceratosa — which nearly resemble one 

 another in external form. S. spiculosa, however, has a skeleton in 

 which the spicules are large and extremely numerous, greatly exceed- 

 ing in quantity the spongin and forming by far the most important 

 element in the skeleton. S. procumhens has a well-developed, rectan- 

 gularly meshed skeleton in which the spongin forms the principal part, 

 but spicules are still distinct and abundant, especially in the primary 

 fibres. S. ceratosa has a skeleton which is to all intents and purposes 

 entirely composed of spongin, the spicules being excessively small and 

 slender and reduced to the merest vestigial structures imbedded in the 

 stout horny fibres. These vestigial spicules are more numerous in the 

 primary than in the secondary lines of the skeleton, whence it appears 

 that the spicules first vanish from the secondary fibres. 



Here then we have a beautiful series of closely related species, 

 all belonging to the same genus , which clearly demonstrate how a 

 siliceous skeleton, with scarcely any spongin present in it, may be 

 gradually replaced by a horny skeleton containing only the merest 

 traces of spicules. 



But this is not all, for in the case of two other species of Chali- 

 mnae, viz. Spinosella plicifera and Spinosella maxima, some specimens 

 of the same species may sometimes contain vestigial traces of spicules 

 while others are entirely destitute of spicules (so far as could be as- 

 certained by careful examination) . Hence it appears that one and the 

 same species may sometimes be a horny sponge and sometimes a sili- 

 ceous one! 



In view of these facts I think no one can doubt that some horny 

 sponges at any rate are descendents of siliceous Chalininae. 



Melbourne, October 25, 1889. 



5. Ein moderner Theoretiker und seine IVIethodil<. 



Zur Abwehr gegen Herrn Rabl. 

 Von R. S. Bergh in Kopenhagen. 



eingeg. 9. December 1889. 



Eine kürzlich unter dem etwas großartig klingenden Titel »Theorie 

 des Mesoderms«^ erschienene Abhandlung von Herrn C. Rabl veran- 



1 Morpholog. Jahrb. 15. Bd. 2. Hft. 1889. 



