THE president's ADDRESS. 241 



are taking up silica in solution from the accessory silicoblast 

 and passing it into the space between the sheath and the spicule. 

 The walls of the three pockets presently become highly refrin- 

 gent (fig. 5), suggesting that solid silica is deposited in them at 

 some distance from the shaft, to form knobs which become 

 connected with the shaft later on. These views are strongly 

 confirmed by the examination of corresponding stages in 

 Latrunculia apicalis (figs. 14, 15), although, as already stated, 

 the formative cells themselves have not yet been observed in 

 this species. 



The development of the spicule up to this point is not, however, 

 exactly similar in the two species, but shows certain well-marked 

 differences which are of great interest from the point of view of 

 the vibratory theory. In accordance with this theory the 

 maximum deposition of silica takes place at the nodes or points 

 of rest of the vibrating rod. In Latrunculia apicalis three 

 thickenings usually appear simultaneously, one at each end and 

 one approximately in the middle (figs. 10-14). The two end 

 ones may appear before the middle one (fig. 9), though this seems 

 to be very rare, but I have never seen the middle one present 

 without the two end ones, and the two end ones are usually of 

 approximately equal size. In L. hocagei, on the other hand, one 

 of the terminal thickenings (the apical one) and the median 

 thickening both appear before the basal thickening (manubrium) 

 is recognisable at all (figs. 2 and 3), and it is a highly significant 

 fact that the median thickening is shifted quite distinctly towards 

 the apex — that is, towards the weighted end of the rod — which 

 Professor Nicholson tells me is exactly what would happen if this 

 thickening represented a nodal point. 



In both species the subsidiary thickening now appears, 

 usually between the median and apical thickenings, but occa- 

 sionally, in L. apicalis at any rate, between the median thicken- 

 ing and the manubrium (fig. 17). Now if these subsidiary thicken- 

 ings also represent nodes there should be two of them, one on 

 each side of the median thickening, and we have to account 

 for the fact that there is only one. The solution of this diffi- 

 culty becomes obvious when we consider the arrangement of 

 the formative cells in L. hocagei. There should be altogether 



