32 HOLASCELLA TARAXACUM. 



at the end into two branches (Plate 22, fig. 8). The rays are 100-160 m thick 

 at the base; the longitudinal ones are 6-22.5 long, the transverse ones 2-10, 

 the distal one 1.5-2.5, and the proximal one about 1 mm. 



In the proximal part of large rays a homogeneous central part, about 40 n 

 thick, and a conspicuously stratified superficial part can usually be distin- 

 guislied. In the axis of the distal part of such large rays structures are observed 

 somewhat similar to those described above in the corresponding spicule-rays 

 of Holascus edwardsii. The axial thread is quite thin in the proximal part of 

 the ray; in the distal part it is considerably thickened, and interrupted by caps 

 composed of a substance of different refractive index from the axial thread and 

 the silica-layers surrounding it (Plate 23, fig. 1). These caps are usually 4-6 n 

 broad and so situated that the convex side lies distally. These caps are irregu- 

 larly distributed along the axis and are very numerous. Sometimes quite a 

 number of them follow in close succession. From the margin of most of these 

 caps a distinct limit between successive silica-layers arises. These limits extend 

 proximally, are conic in shape, and pass uninterruptedly into the limits between 

 the silica-layers forming the outer, clearly stratified zone of the proximal part 

 of the ray. These limits represent former surfaces of the spicule, whilst the caps 

 mark the positions of the tip of the ray at various times. There can be little 

 doubt that here, as in Holascus edwardsii, the growth of these spicules is intermit- 

 tent, interrupted by periods of rest. Every time the longitudinal growth of the 

 rays recommences after such an interruption a cap is formed. 



It has been stated above, that in some of the large principal hexactines one 

 of the rays is reduced in length and terminally thickened. In the centre of the 

 terminal thickening of such shortened rays the central, unstratified zone of the 

 spicule ends in the shape of a slender, pointed cone. The terminal thickening 

 itself is formed exclusively by the clearly stratified superficial zone, each layer 

 of which is here markedly thickened. 



The few large principal pentactines and tetractines (Plate 22, figs. 7, 8, 11) are 

 similar to the principal hexactines described above. Most of them differ from 

 the latter only by the absence of one (the pentactines) or both (the tetractines) 

 the radial rays. In some of them also the difference of the longitudinally and 

 transversely extending rays is less pronounced than in the principal hexactines. 



The axes of the intermediate hexactines are not differentiated and, although 

 the rays are in the same spicule often more or less unequal, they are apparently 

 equivalent. The rays are 140-300 fi long, usually cylindrical, 7-12 n thick, and 

 rounded and often thickened at the end. The tips of the rays are spiny. The 

 other parts of the spicule are smooth. 



