The Skeleton and Classification of Calcareous Sponges. 71 



attained by the passive submission of a solution of carbonate of lime 

 to forces of crystallisation ; thus, with economy to the organism, very 

 simple secreting cells produce a result similar in effect to that which 

 would be attained by morphographic skeletogenous tissues of far 

 more complex heredity. It would appear from comparison of the 

 spicules of Sycon and Leucascus, Anamixilla and Heteropegma (vide 

 infra), that in calcareous sponges this advantage completely out- 

 weighs any possible profit or loss resulting from slight change in 

 angle or substitution of curved for straight rays. 



It is difficult to believe that the equiangular spicule with which 

 Seteropegma and Leucandra australiensis support their tubes, alike 

 has induced their survival, and would cause death to the Sycon that 

 should imitate them. Nor, on the other hand, can the spicule forms 

 of 8. raphanus be ascribed to a rigid atavistic heredity ; for between 

 individuals self-sown in the same tank, the striking differences of 

 homologous spicules proclaim their form to be most variable. The 

 explanation of the exact geometrical figure observed in any one case 

 is to be sought in crystallography, and not in physiology; in the- 

 laws which we recognise as governing form in dead matter, that is,, 

 those that are apparently independent of any forces in the dimension 

 of memory. 



So far as can be judged, it seems that the change in form, which, 

 through influence on the direction of the optic axis, results from 

 change of stress, renders the spicule less well adapted to bear that 

 stress ; a predominant longitudinal tension placing the paired rays 

 nearly transverse to its own direction. A priori, where one cha- 

 racter in an organism is physically a function of another character in 

 the organism, the variation of one only can be exactly correlated ta 

 the needs of the organism. If both are important, the adaptation of 

 neither can be perfect; according as either is predominant, the 

 other exhibits phenomena which do not conduce to survival of the 

 variety in which they are noted. 



The calcite crystal may be compared to a symbiotic organism ; its- 

 characters within certain limits of saliency are subordinated to the 

 needs of the organism within whose tissues it finds a welcome. But 

 the number of mesenteries in Adamsia is dictated by its own history, 

 and not by the mode of life of the crab which carries it; so the 

 angles of a triradiate calcareous spicule are dictated by the proper- 

 ties of calcite, and, within a considerable range, would appear neither 

 to influence nor be influenced by selective mortality in the species 

 among which it occurs. To hold that this view is unjustified it> 

 seems necessary to suppose that individuals of A. cerebrum, in whom 

 gastral thorns were not parallel to the facial rays of the spicule 

 which bears them, have consequently perished and been unable to> 

 procreate their race. July 23, 1898.] 



