68 THE VOYAGE OF H.M.S. CHALLENGER. 



beams are crossed on the one hand by long spiral fibres, which run parallel to the edge 

 of the ridges, and on the other hand by small short beams, which run through the 

 ridges transversely. The ridges are doubtless to be regarded as folds of that part of 

 the tube-wall which extends over those meshes of the inner quadrate lattice-work 

 not occupied by parietal gaps, and which is supported by spiral fibres crossing one 

 another obliquely. The long ridge-fibres running parallel to the margin are to be 

 looked upon as a direct continuation of one of the two systems of spiral fibres, which 

 cross one another obliquely, and further, the rafter-like beams which intersect the 

 former at right angles are to be ascribed to the other system of those spiral bands. 

 The latter appear in the ridges as if laid in a fold sharply involuted on the outer 

 extremity (PI. II. fig. 8). 



The strong circular and longitudinal fibrous bauds, which are so manifest on the 

 inner side of the macerated tube, have for their groundwork the much drawn-out, strong 

 rays of the regular, simple and cruciform tetracts. These are interwoven in a peculiar 

 manner, and become subsequently firmly united by cementing matter and synapticula. 

 While all the circularly arranged rays of these tetracts run along the inner side of the 

 tube, the longitudinal rays on the other hand lie transversely across the outside, and all 

 the rays extend over several adjacent tetracts, thus producing the interlacing which is 

 suo-o-ested in PI. II. fig. 2. The long siliceous fibres which form the basal tuft join the 

 longitudinal beams of the quadrate network externally, from the commencement of the 

 inferior third portion of the tube. 



The narrow, much prolonged comital spicules, which are closely applied by the two 

 long rays of their principal axis, partly to the circular rays and partly to the longi- 

 tudinal rays of the large tetracts, and which at a later period become fused together, are 

 for the most part triacts (PI. II. fig. 4 ; PI. III. fig. 12), whose unpaired ray is continued 

 into the oblique spiral bands of the tube-wall, or contributes to the formation of the 

 ridges. More rarely the comital spicules are diacts or irregular tetracts, in which the 

 longer rays lie in one and the same axis, while the two other shorter rays are placed at 

 right angles to one another (PI. III. fig. 20). 



The oblique spiral bands of fibres which extend over the covered meshes of the 

 quacbate network, and the firm margins of the parietal gaps, are chiefly composed of the 

 long principal axes of triacts. Here and there irregular tetracts also occur, and diacts 

 whose atrophied transverse axes are usually more or less prominent owing to the 

 presence of prongs. 



Pentacts and hexacts are less frequently found among the thread-like elongated 

 spicules (PL III. fig. 17). 



The extremities of all those needles, which are subsequently firmly united, exhibit 

 remarkable modifications, a few of which are figured on PL III. figs. 2-10. The rays 

 seldom run out to a point, with a gradual decrease of diameter (PL III. fig. 2) ; in most 



