64 



The diagonal spicular fibres form two systems of spirals in the tube-wall 

 crossing each other at right angles. They partly extend outside of the longi- 

 tudinal fibres and partly between the longitudinal and transverse ones and con- 

 sist exclusively of long and thin triactines and diactines with terminally-thickened, 

 club-shaped rays. They are usually arranged in such a manner, that they cut 

 off the corners of the rectangular meshes of the primary network formed by the 

 longitudinal and transverse fibres, leaving the central part of the mesh free, so 

 that a parietal aperture can be formed in it. These systems of spiral fibres form 

 a network with quadratic meshes, which lie obliquely side by side and are in the 

 longitudinal and transverse directions in contact with each other by their corners. 

 In each of these meshes of the spiral fibre-net alternately a mesh-centre and a 

 node of the primary network of longitudinal and transverse fibres are situated. 

 Sometimes branched spiral fibres take a considerable part in the formation of 

 the supporting skeleton of the freely projecting crests and ridges on the outer 

 surface. /^ 



Where the tube-wall passes into the terminal sieve-plate, which is nearly 

 vertical to it, the stout stauractines are replaced by nearly equally thick, smooth 

 triactines, pentactines or even unilaterally developed hexactines. One ray of these 

 triactines extends longitudinally in the tube-wall, whilst the others, which diverge 

 at right angles from it, lie in the margin. Three of the rays of the pentactines are 

 arranged similarly to the rays of the triactines, the fourth ray extends radially 

 and distally into the collar and the fifth ray radially and proximally into the 

 terminal sieve-plate. The hexactines are similar in shape and position to the 

 pentactines and differ from them only by the possession of a short, terminally 

 thickened and club-shaped sixth ray, which lies opposite to the longitudinal ray 

 extending downwards in the tube-wall. These principal supporting spicules of 

 the margin are also accompanied by numerous thin, triactine and diactine comi- 

 talia. • 



All these supporting spicules can be firmly united by secondarily apposed 

 masses of silica. This joining takes place in all the older parts of the body where 

 the rays of adjacent spicules approach each other closely, parallel rays lying very 

 near each other being usually joined by a series of synapticula so as to form 

 ladder-like structures. This joining or glueing process advances in accordance 

 with the age of the parts from the base of the sponge upwards. The funnel- 

 shaped lower end of the body, which lies hidden in the midst of the upper part 

 of the root-tuft, is consequently already supported by joined fibres and is hard 

 and resistant at a time when the central and upper parts are still supported by 

 isolated spicules only and consequently soft and resilient. 



The spicules supporting the terminal sieve-plate are not joined by silica and 

 differ considerably from the supporting spicules of the tube-wall. They are 

 oxydiactines, angularly bent in the middle, and generally possess a more or less 



