32 
HOLASCELLA TARAXACUM. 
at the end into two branches (Plate 22, fig. 8). The rays are 100-160 n 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- 
guished. 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 edivardsii. 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 m long, usually cylindrical, 7-12 ^ thick, and 
rounded and often thickened at the end. The tips of the rays are spiny. The 
other parts of the spicule are smooth. 
