40 ART. 7. — I. IJIMA : HEXACTINELLTDA, IV. 



either isolatedly or in weak bundles. They are all tliin and 

 filamentous, probably never attaining a thickness of more than 

 12 ij.. The majority, if not all, of them have the spicular center 

 externally indicated by an annular s\yelling or by four cruciately 

 disposed knobs. The rays are smooth except at the roughened 

 ends, which sometimes terminate conically and at other times 

 taper out to a point. Scarcely any of the diactins can l)e dis- 

 tinguished as principalia. 



The latter are l)y no means uncommon, though they are 

 more plentiful in some individuals than in others. They some- 

 times appear to be comjmratively strong. Thus, a large parenchy- 

 mal hexactin may measure 1.6 mm. in axial length, the rays 

 being 30 // thick at base. However, the majority of the hexact- 

 ins are considerably smaller and weaker. The rays gradually 

 taper outwards and are smooth all over except near the conically 

 pointed end. As a general rule the hexactins are so disposed 

 that one of the axes is radially directed, without however show- 

 ing much regularity in the arrangement. Sometimes their rays 

 are seen to pursue a solitary course ; moi-e frequently are they 

 joined with the diactins in small numbers to constitute the par- 

 enchymal strands. 



As in so many other lyssacine Hexactinellids, the basal end 

 of the sponge exhibits a typical basklictyonal mass (PI. III., fig. 

 22). This consists of a rigid, irregularly meshed reticulum of 

 comparatively thick l)eamä, the surface of which is beset through- 

 out with small tubercles. These beams, for the most part, may 

 without difficulty l^e recognized as the rays of unusually stout 

 hexactins, which are ankylosed either directly ray to ray or by 

 means of synapticular formations. The synapticulse on the surface 



