70 W. WOODLAND. 



Sollas and the alveolar theory of Dreyer are notable examples. 

 With reference to the latter^ the inability of the theory to 

 account for the well- ascertained fact that all spicules arise 

 from approximately spherical concretions, and that com- 

 paratively very few ever assume the tetraxon form, releases 

 us from the necessity of considering it.^ The theory of 

 Sollas, on the other hand, logical enough in its premises, and 

 possibly providing an explanation of some forms of spicules, 

 most lamentably fails when confronted, e. g. with the com- 

 plicated siliceous spicules of many sponges and the calcareous 

 spicules of many holothurians. Sollas's theory, however, if 

 properly understood and applied, accounts for a good many 

 of the facts. Consider, e. g. the significant fact that the 

 forms of spicules in general are constantly adapted as regards 

 their space dimensions to their position in the organism con- 

 taining them ; flat spicules, e. g. are generally found in 

 situations limited by two more or less definite parallel 

 surfaces (triradiates of Calcarea, plate-spicules of holothurians, 

 etc.), and spicules of three dimensions are always found 

 either in situations far removed from limiting surfaces (most 

 alcyonarian spicules, asters of siliceous sponges, and some 

 colonial ascidians, etc), or with their parts disposed in 

 relation to these (quadriradiates of Calcarea, the hexactine 

 macrosclere and its modifications in hexactinellids, etc.) . More- 

 over, definiteness of form of the spicule generally, if not 

 always, exhibits some correspondence with the definiteness of 

 its immediate enviroment. Most symmetrical spicules occur 

 in regions of the organism which are symmetrically disposed 

 with regard to the spicule, and irregularly-formed spicules, 

 on the other hand, are generally found in situations charac- 

 terised by the absence of definite arcliitecture. Such corre- 

 spondences, to be found both in spicules contained in different 

 organisms and in different parts of the same organism, must 

 be due to the effects of either factor (a) or factor (?;) on the 



1 Dreyer's similar explanations referring the forms of radiolariau shells to 

 an alveolar conformation of the scleroplasm have no facts whatever to support 

 tliem. There is no evidence of this particular alveolar conformation. 



