Potash on the Siliceous Skeletons of Sponges. 299 



which the fibre has been formed, and afterwards enlarges and 

 unites these together so as to form a continuous canal in the 

 axis of the fibre. 



4. The '' rosettes " and other flesh-spicules of the Hexacti- 

 nellids pass into solution before the hollow sexradiate casts 

 have been definitely produced. This explains the fact that 

 these spicules have never yet been observed in the fossil state. 



5. The spines and tubercles of vitreo-hexactinellid fibre 

 persist up to a very late stage of solution, and considerably 

 outlast the rosettes and flesh-spicules. 



6. Except in the case of Eu^ilectella^ the vitreo-hexactinellid 

 fibre does not exhibit a tendency to scale oft' into concentric 

 layers when undergoing solution. 



7. In ordinary spicules the caustic potash insinuates itself 

 between the faces of the component lamellte, which thus dis- 

 solve not only at their edges, but over their whole surface, 

 and in consequence become separated from each other and 

 so display the true structure of the spicule. 



8. The globo-stellate spicules dissolve chiefly from within 

 outwards : a central cavity is formed with hollow rays pro- 

 ceeding from it ; this cavity continuously enlarges towards the 

 exterior till the siliceous substance surrounding it becomes 

 reduced to a mere film. 



The unbuilding of these spicules thus proceeds exactly in 

 the same way as their building-up ; the growing hollow cavity 

 with its rays is an exact negative image of the solid sphere 

 with its fine projecting spines that passes at length into the 

 adult globo-stellate form. 



9. The solution whicli suffices to analyze the various layers? 

 of a large spicule from each other destroys altogether small 

 spicules, such as those oi Halichondria panicea. 



10. The external solution of the spicules may take place 

 with comparatively greater rapidity at some points than others 

 on the surface, which thus becomes excavated by a number of 

 hemispherical pittings. 



11. Dead spicules are soluble also in water containing car- 

 bonic acid ; and thus arises a correspondence in the characters 

 of spicules artificially treated with caustic potash and those 

 which are dredged up from the sea-floor or obtained in a 

 fossil state. 



EXPLANATION OF PLATE LX. 



Alterations produced iii spicular structures by boiling in caustic potash 



solution. 



Fig. 1. Dactylocahjx suhfjiobostis. Skeletal network, exhibiting casts of 

 sexradiate spicules (xl04). 



