231 



THE PRESIDENT'S ADDRESS. 



THE CHESSMAN SPICULE OF THE GENUS LAT- 

 RUNCULIA; A STUDY IN THE ORIGIN OF SPECIFIC 

 CHARACTERS. 



By Arthur Dendy, D.Sc, F.R.S. 

 Professor of Zoology in the University of London. 



{Delivered February 21th, 1917.) 



Plates 15-17. 



It is very well known that there are certain types of tetraxonid 

 microscleres in which the spicule consists of an elongated, rod- 

 like axis with whorls of outgrowths arranged at intervals along 

 it. As a result of the study of some novel varieties of this type 

 met with amongst the sponges of the " Sealark " collection, 

 it occurred to me that the position of these whorls might corre- 

 spond with that of the nodes in a vibrating rod, and that thus 

 the form of the spicule might be determined largely by a well- 

 known physical cause, the deposition of silica taking place 

 chiefly where the rod was in a state of rest. The stream of water 

 constantly flowing through the canal- system of the sponge seems 

 to afford a sufficient explanation of such vibration, for the 

 developing spicules are imbedded in the very soft, gelatinous 

 mesogloea in the immediate neighbourhood of the canals. I 

 submitted the problem to my mathematical colleague, Professor 

 J. W. Nicholson, of King's College, and he was good enough to 

 make some calculations which entirely support this theory. I 

 do not propose to trouble you on this occasion with a detailed 

 account of our results from the physical point of view, as we 

 hope to publish a joint paper on the subject very shortly. In 

 searching for further evidence, however, I was led to make a 

 somewhat elaborate study of the so-called chessman spicule in 

 two species of the genus Latrunculia, and the development of 



