50-2 



THE VOYAGE OF H.M.S. CHALLENGER. 



110 spicules occur, and only iiuich modified elements in the two limiting lamella;, the 

 basis, the oscular margin, &e. 



If we assume that the spicules in the body- wall serve essentially only for the support 

 or stiifening of the soft parts, it is to be expected that that form and disposition of the 

 skeletal elements will in eacli case be developed which in the given circumstances is 

 best fitted to give the necessary firmness to the body-wall. 



I am decidedly of opinion that it can be shown with convincing probability that 

 such a necessary relation does exist between the structure of the soft parts in each of 

 the three principal groups of sponges and the characteristically typical forms of spicule 







rQf 



Fic. 10. — Triacts with each of the three rays lying at 

 a uniform angle between two adjacent pores. 



Fig. 11.— Triacts di-sposed so that half of the interspaces 

 are occupied by their central portions and tlie other 

 lialf by their convergent rays. 



which we regard on anatomical and developmental ground as primitive and fundamental 

 for each group. 



If a plate is to be perforated by the maximum number of uniformly large spherical 

 pores in such a way that the lumina of the pores have a certain scope for expansion or 

 contraction, these pores can only exhibit one definite mode of arrangement, namely, that 

 of the cells of a honeycomb, and will leave a network with somewhat broad beams 

 between them. 



If the plate consist of a mass which requires to be supported by the deposition of 

 hard parts, and if these are, on the one hand, to preserve the maximum of firmness, and on 

 the other to allow of a certain degree of expansion both to the entire tube and also to the 



