Vibrations upon the Form of Certain Sponge- Spicules. 587 



ontogeny, and it will be of great interest in the future to see whether this 

 factor has played any part in the evolution of organisms outside the limits of 

 the Phylum Porifera. We do not propose to speculate upon this subject, but 

 it may be well worth while for botanists and zoologists to keep a look out for 

 possibilities in this direction. No doubt the sponges are especially likely 

 to exhibit cases in which vibrations play an important part, because the 

 water-currents which flow through their canal system with considerable 

 strength must tend to set up such vibrations in any elastic bodies of suitable 

 shape embedded in the almost liquid mesogloea. But even amongst the 

 sponges, the demonstrable cases, so far as is yet known, are con fined to 

 members of the single sub-family Spirastrellinse, belonging either to the 

 genus Latrunculia or to a closely related genus. It is by no means 

 impossible, however, that the form of certain other sponge-spicules may have 

 been influenced by vibrations during development, though we are inclined 

 to think that in most cases the action of surface tension is much more 

 likely to afford a profitable field of investigation. In the case of the 

 oxydiscorhabd, however, it does not seem possible to account for the position 

 of the whorls except by the vibratory theory, and it seems highly probable 

 that the same is true of the Latrunculia discorhabd, although the latter has 

 not been submitted to exact mathematical analysis. 



The Lateral Vibrations of Bars of Variable Section. 

 By J. W. Nicholson, M.A., D.Sc, F.E.S. 



[This paper, arising out of the foregoing, will appear in Series A of the 



' Proceedings.'] 



