508 On the Hexaciine Spicule of Hexactinellida. 



tlivee-dimensioned structure are dispensed with, and the one- 

 dimensioned prop or standard (amphidisk) is brought into 

 requisition. Although the amphidisk is probably a reduced 

 heXadisk, the axis-cross representing the aborted rays is not 

 in evidence, so completely has the triaxial character been 

 suppressed. 



1£ the coincidence of the primary axes of the spicule with 

 those of the regular system can be accounted for on grounds 

 of selection of a form adapted to maintain the patency of a 

 mesbwork, certain cases of coincidence with secondary planes 

 of symmetry are easily explained. 



The truemicrohexactin of Hyalonema divergens, F. E. Sch., 

 and the monoxyhexaster of Bathydorus uncifer are both of 

 approximately the same shape, i. e. with rays meeting at 

 right angles in a common centre, and with curved ends 

 lying in secondary planes of symmetry. In the case of 

 Hyalonema^ the spicule is a true microhexactin with axial 

 canals ruiniing to the very points of the rays ; in the 

 monoxyhexaster, the axial canals only extend a short distance 

 from the centre. Along with the monoxyhexasters are hemi- 

 oxyhexasters with some main rays ending in more than one 

 end ray ; and there is no reason to doubt that the monoxy- 

 hexasters are reduced from such forms, and that the curved 

 ends are merely deflected spines or end rays. Whatever theory 

 one may adopt concerning the micro-hexactins of Hyahnema, 

 it is difficult, in view of the probable history of the spicules, to 

 believe that the incidence of the ends of the rays of the mon- 

 oxyhexaster of B. uncifer m secondary planes of symmetry is 

 anything more than the result of the stresses and strains of the 

 strands of contractile meshwork. Similarly, as Schulze has 

 shown, the pointing of the rays of the discoctaster to the angles 

 of a cube simply results from centripetal pressure suppressing 

 the main rays of a hexaster and pressing back the sclero- 

 blastic end rays (2, 3, 4 or many) till they fuse with 

 neighbouring rays into secondary main rays ; this incidence 

 of axes in lines pointing to angles of a cube is a pure 

 coincidence; frequently "supernumerary thorns" fail to 

 become fused and do not point to the angles of the cube. 



Summary. Keasons are given for the belief : 



(1) That the regular hexactine spicule was primarily 

 formed in Hexactinellid sponges as being the most econo- 

 mical and efficient * means for supporting the strands of a 

 syncytial network ; for, in the gastrosome at any rate, the 

 microscleres would be useless for upliolding the body or 



* The human architect, also, has found that the regular liexactin 

 is the most convenient form of spicule for constructing his dictronine 



