560 DR. R. VON LENDENFELD ON THE SYSTEMATIC [DeC. 21, 



clature of Hexactinellid spicules is here, of course, accepted en bloc, 

 and his terms will be defined below. Sollas (1453) has used a 

 number of terms in his preliminary report which I do not under- 

 stand and which presumably nobody else understands either. It is, 

 therefore, much to be regretted that the greater part of them are 

 unexplained. In consequence of this I will, for the present, abstain 

 from attempting to compile a nomenclature of Tetraxonid spicules 

 pending the publication of Sollas's full report, in which, we may 

 hope, he will explain his new terms. 



The spicules of Sponges are, as a rule, of such shape that they 

 appear as more or less modified geometrical figures with definite axes. 

 The axes are always represented by a non-skeletal rod (the so-called 

 axial canal), round which the silica or lime is precipitated iu con- 

 centric layers. There may be one such axis, or there may be more 

 than one. 



Ilackel (627 ) drew attention to this crystalline regularity of 

 sponge-spicules, which has been of great importance in studying the 

 skeletal elements of Sponges. 



W^e can divide the sponge-spicules, as we do the Sponges, into the two 

 groups Calcarea and Silicea, according to their chemical composition, 

 \A'ithiii each group we distinguish series of forms according to the 

 number and position of the axes. The validity of this classification 

 is proved by the correlation of these different kinds of spicules with 

 other organs in the Sponges. 



The following are the different kinds of spicules : — 



I. Group Spicula Calcarea. 

 Composed chiefly of carbonate of lime. 



1. Monaxonia. 

 With one straight or curved axis, rod-shaped. 



2. Triaxonia. 



With three distinct axes which may lie in one plane or not. 

 When one of the rays of this tri-act spicule becomes rudimentary, 

 Diaxonia can theoretically be produced. It is, however, advantageous 

 to consider the Diaxon spicules as part of the Triaxonia. The 

 calcareous triaxon spicules have only three rays — triact. 



3. Tetraxonia. 

 With four axes and four rays — tetract. The points form the 

 corners of a triangular pyramid. Generally three axes, or rays, are 

 equivalent (tangential), and one (radial) is differentiated, longer or 

 shorter than the others. 



II. Group Spicula Silicea. 

 Composed chiefly of silica. 



1. Anaxonia. 

 Without definite axes and with numerous rays — polyact. 

 To this group belongs one kind of spicule only, namely the stellate 

 and its derivatives. 



