242 THE president's address. 



three nodes between the two ends of the rod, but there are only 

 two rings of formative cells, and apparently the nodal thickening 

 cannot take place in the absence of such a ring. 



This brings us to another extremely interesting consideration. 

 It must be primarily the position of the rings of formative cells 

 that is determined by the vibrations of the rod, and only secon- 

 darily that of the local accumulations of silica which develop into 

 the whorls of the adult spicule. It would seem that we have 

 here a kind of tropism exhibited by the formative cells. Many 

 different kinds of such tropism, or taxis, are already recognised, 

 such as chemiotaxis, where the position of the cell results 

 from response to chemical stimulation; galvanotaxis, resulting 

 from response to electrical stimulation ; geotropism, resulting from 

 response to the stimulus of gravity, and so on. It seems only 

 reasonable, on the evidence before us, to suppose that the 

 formative cells are sensitive to vibrations in such a way that they 

 are induced to avoid the internodes and take up their positions 

 at the points of comparative rest of the vibrating rod, with 

 which they are doubtless at first in immediate contact. 



We have noticed in describing the form of the adult disco- 

 rhabd in our two species that the chief specific distinction lies 

 in the fact that in Latrunculia hocagei (fig. 7) the apex is broadly 

 rounded and provided with a number of capitate spines, while 

 in L. apicalis (figs. 20, 21) it is drawn out into a long, slender, 

 tapering end-piece. This difference is obviously to be attributed 

 to the different behaviour of the protorhabd during development 

 in the two cases. In Latrunculia hocagei the protorhabd appears 

 to have finished its growth completely by the time that the 

 deposition of silica commences, so that first the apical and then 

 the basal extremity becomes completely enveloped. Growth in 

 length of the spicule as a whole is no longer possible except for 

 the very slight amount due to the thickening of the silica deposit 

 at each end. In L. apicalis, on the other hand, the protorhabd 

 continues to grow apically for a long time after all the thickenings 

 of the shaft have made their appearance. Apparently the original 

 apical thickening never completely envelops the end of the 

 protorhabd, which goes on elongating until it has become about 

 twice as long as it was when the deposition of silica first com- 



