574 , Profs. A. Dencly and J. W. Nicholson. Influence of 



position of these whorls is determined by the fact that the spicule, at the 

 time of their commencement, is in a state of vibration, clue to the water 

 currents flowing through the sponge, the whorls corresponding to the nodes 

 or positions of comparative rest. The special accumulation of silica on the 

 nodes appears to be due, not directly to the vibrations of the spicule, but to 

 the fact that the formative cells exhibit a kind of tropism which induces 

 them to settle down and perform their work in the positions where they are- 

 least disturbed by the vibrations. 



The whorls in this case are not sharply defined at the moment of their first 

 appearance, so that it is impossible to obtain accurate measurements for 

 mathematical analysis ; nevertheless, there are certain facts connected with, 

 their arrangement which, in our opinion, afford a fairly conclusive demonstra- 

 tion of the view that they are deposited approximately upon the nodes of a 

 vibrating rod. Two species were investigated, Latrunculia apicalis and 

 L. hocagei. In both species the spicide, at a certain stage of its developments 

 consists of a straight rod with four thickenings, representing a basal 

 manubrium and three incipient whorls. There is a basal thickening at one 

 end, an apical thickening at the other, a median thickening at or near the 

 centre, and a subsidiary thickening, usually between the median and apical 

 thickenings, but occasionally between the median and basal thickenings. If 

 these thickenings correspond to nodes, we have to account for the fact that a 

 .subsidiary thickening is developed only on one side of the median thickening. 

 The solution of this difficulty is to be found in the arrangement of the 

 formative cells (observed in Latrunculia bocagei only, though doubtless 

 occurring in the other species also), for while there is a ring of formative 

 cells round the median thickening and a similar ring round the subsidiary 

 thickening, there is none around the part of the spicule where a second 

 subsidiary thickening might be looked for, and hence no whorl is developed 

 in this situation, in spite of its being a nodal point. No formative cells have 

 yet been observed in relation to the basal and apical thickenings. 



Certain differences in the development of the discorhabd in the two species 

 of Latrunculia dealt with are of the highest significance from the point of 

 view of the vibratory theory. In L. apicalis the young spicule is symme- 

 trical, the basal and apical thickenings being represented by rounded knobs 

 of approximately equal size, while the median thickening, consisting typically 

 of a whorl of three knobs, appears just about half-way between them, 

 developing either simultaneously or (perhaps always) a little later. In 

 L. bocagei, on the other hand, the apical thickening appears some time before 

 the basal thickening, so that we have an asymmetrical rod weighted at one 

 end, and the median thickening is actually shifted towards the weighted end 



