500 THE VOYAGE OF H.M.S. CHALLENGER. 



0. Schmidt has also occupied himself with the notion of explaining the form of the 

 typical sponge spicules in terms of the crystallising tendencies of the different substances. 

 He says : " The question is a difficult one, how far the nature of the lime and the flint 

 determines the rayed and anchor-shaped forms related to the three-sided prism. For 

 the flint the explanation holds good, but not for the lime. But while in the group 

 of triaxial siliceous spicules (hexacts and their derivatives) we have only to choose 

 between the triaxial and the binaxial and monaxial system, and the hexagonal does 

 not come into account, we must remember the fact, inconvenient to mineralogists, that 

 quartz crystals arising in amorphous matrix not unfrequently exhibit axial deviations from 

 the hexagonal system, and that we may the more readily expect in our sponge spicules 

 with organic basis and admixture other forms than those of the crystallographic systems." * 



For my own part I must pronounce against such an attempt to bring the form of the 

 sponge spicules, whether they consist of carbonate of lime or hydrated silica, into relation 

 with the crystallising tendencies of these substances in the way of origin or explanation. 

 In the first place, in regard to the siliceous spicules, the fact has to be noted that the 

 silica in them never occurs in a crystalline state, but is always present as completely 

 amorphous hydrated silica or opal, as is shown, for instance, by the fact that they are not 

 doubly but only simply refractive. This is therefore against the supposition that the 

 various skeletal elements can be reduced to or derived from the crystallisations of the 

 substances of which they consist. Furthermore, we cannot ignore the extraordinary 

 abundance and frequent importance of the deviations of the rays from the typical angle 

 at which they ought to stand to one another, nor does the marked curvatures of the rays 

 accord with the supposition of defined crystalline axes. 



I must rather maintain that the form of all the sponge spicules is determined by the 

 organic matrix in and from which they originate, and that the formative forces are in no 

 essential way different from those Avhich are everywhere exhibited in the shaping of the 

 living organism and its parts. 



And though we still know very little about the forces determining form, yet in these 

 skeletal formations it is possible here and there to detect factors which, though they do 

 not indeed explain everjrthing, yet make much at least more intelligible. 



If one can distinctly demonstrate a natural and necessary connection between 

 the form and disposition of a skeletal element and the function which it discharges, one 

 has, from the standpoint of utility and natural selection, rationalised the appearance of that 

 form and disposition. 



In regard to the question why the typical and primitive spicule in the calcareous 

 sponges should be the plane, regular, triradiate form, in the Tetraxonia with their 



• 1 Gruudziige einer Spongienfauna des atlantiscben Gebietes, p.- 4. 



