447 



this hypothesis would require. In all recent Hexactinellids the trahe- 

 culae are disposed ([uite iiTegularly. without any dehnite pattern. The 

 second explanation Avhich seems to me not only possible, but highly 

 probable, is that the symmetry of the stauractine and other forms of 

 triaxon spicules was due in the first instance to the inherent properties 

 of the material of which they were formed. In a brief but highly sug- 

 gestive memoir Schulze [11] has pointed put how exactly the sym- 

 metry of the axes of the triaxon sjiicule, in all its diverse forms, cor- 

 responds with the axes of crystals of the cubic system. As I have said 

 elsewhere [7, p. 120], the many startling coincidences adduced by 

 Schulze invite a renewed investigation of the physical nature of the 

 material of which the spicules are formed. Such an investigation might 

 give a clue to the origin of the symmetry of the triaxon spicule, and 

 explain from what cause the primitive stauractine derived the constant 

 right-angles at the junction of its rays, in spite of the fact that it took 

 origin, apparently, under circumstances in which no adequate biological 

 reasons can l)e adduced to explain the constant and characteristic sym- 

 metry. 



I may now sum up briefly, in conclusion, my views upon the phy- 

 logeny of the Hexactinellida in the following propositions : — 



1) In the ancestral form of the Hexactinellids, and perhaps of all 

 sponges, the gastral layer was in the form of a continuous sheet of 

 collar-cells suspended evenly in the midst of the dermal layer, which 

 formed a trabecular system developed as much towards the interior as 

 towards the exterior of the gastral layer. 



2) The spicular skeleton arose first in the trabecular system situ- 

 ated externally to the gastral layer, and the earliest regular form of 

 spicule was the stauractine. This stage of evolution is represented by 

 the palaeozoic Stauractinophora of Schrammen, especially by the 

 Protospongidae. 



3) The next step in phylogeny was the folding of the gastral layer 

 to form distinct flagellated chambers, and with this change the staur- 

 actines developed additional rays directed radially, thus producing the 

 hexactines found in all Hexactinellida after the palaeozoic epoch, and 

 probably also in many even at that early time. 



List of Works Quoted. 



1) Dendy , A., The Organisation and Classification of the Calcarea Homocoela etc. 



Trans. K. See. Victoria. III. p. 1—81. pis. 1—11. 

 2j Ha e ok el, E., Die Kalkschwämme. Berlin, 1872. 3 Vols. 

 3) Ijima. I.. Studies on the Hexactinellida. I. Joiirn. Coll. So. Tokyo. XV. 1901. 



299 pp., 14 pis. HI. ibid. XVIII. 1903. 124 pp.. 8 pis. IV. ibid. 1904. 



307 pp., 23 pis. 



