MATERIALS FOR A MONOGRAPH OF THE ASCONS. 565 



being so^ it is difficult to see how the functional excitation, 

 so feasible in pliylogeny, can influence the ontogeny, since 

 the tiiiadiate systems have their characteristic and definite 

 form long before they are able to exercise their function of 

 support. The adaptation of their form to the structure of 

 the sponge is therefore entirely prophetic. The only way in 

 which functional excitation could modify their form would 

 be to produce curvature of the rays at a late period of their 

 growth. Such a curvature is found normally in the spicule 

 rays of Leucosolenia, and exceptionally in Clathrina, and 

 is perhaps the result of a functional excitation exercised as 

 follows. If we look at a young triradiate system surrounded 

 by its six cells (compare especially PI. 38, fig. 8, PI. 39, fig. 21, 

 and PI. 41, fig. 38), it is impossible to avoid the idea that the 

 apical formative cell exerts in some way a directive function 

 in laying down the ray. Going ahead at the extreme tip of 

 the ray, it seems to mark out its future direction. Of the two 

 formative cells, the basal cell might be termed the secretive 

 cell, the apical cell the directive element. If now an apical 

 formative cell, acting in this way, were deflected from its path 

 and turned to one side by a pore or other tissue element, a 

 curved ray would result. I hope at some future time to be 

 able to bring forward observations on this point, and especially 

 to trace the history of abnormal spicules, and investigate the 

 causes of their abnormality. In any case the functional 

 adaptation could only produce, as has been said, a late curva- 

 ture of the rays, and could not influence the fundamental 

 angles at which they meet, for the latter are determined at a 

 very early period within the secreting cells. The apical for- 

 mative cell might represent a medium for auto-regulation 

 and ontogenetic adaptation, but in the case of spicules which 

 lose their apical cells at an early period, variations in the 

 curvature of the rays due to a functional stimulus would be 

 impossible. The spicule must, it would seem, in such cases 

 grow straight on, like the incisor of a rodent, which grows 

 into its brain if not worn down, and if a pore is in the way of 

 the spicule it is the pore which must shift its position. 



VOL. 40, PART 4.— NEW SEE. R R 



