204 
7r ins sic Echinoderms of Bnkony. 
number of irregulär, more or less equivalent septa, having a general tendency to 
radiate from the centre; this regularity and centrifugal arrangement increases towards 
the periphery, vvhere also the septa become markedly closer and finer, about 75 to 
the millimetre, with the meshes between them smaller and more regulär. Thus far 
the description might apply to such a form as Cidaris dorsata, or to C Waechteri 
were it not for the axial complex of that species ; but the obvious difference lies 
in the grouping of these septa into a number of fan-systems. The starting-points 
of these are marked in the section by darker patches, which are not easily 
explained. Some of these dark patches are seen near the centre, but in that 
region the fan-systems are developed very slightly or not at all. In the peri- 
pheral region also no fresh fan-systems seem to originate, but the septa arising 
from those in the deeper layer continue and dichotomise, and fan out in the ridges 
in a manner that is quite usual. In this peripheral region, at a little distance from 
the exterior, is a dark band, which seems to coincide with a region of dichotomy 
and consequently of closer septa. It may be that, with the further growth of the 
radiole, this band would break up into nuclei of fan-systems, each System marking 
the former position of a ridge. Now, if the ridges were regulär and always stayed in 
the same relative position, the fan-systems would form a continuous series along a 
radial line. Possibly in a young radiole, or in a section near the base, this would 
be the case, and the general appearance would no doubt then approach that indi- 
cated in Dr. Hesse’s diagram. But, as we have seen, the ridges in the adult shaft 
are not regulär: they wave and are separated by newly-formed ones and again 
coalesce. Probably, then, they shift their relative positions during growth, and so 
successive fan-systems cease to lie in a straight radial line. Resorption and rede- 
position of stereom, in the manner well known among Echinoderms, probably take 
place in the deeper layers and further obscure the supposed original symmetry. 
Thus it may be possible to harmonise Dr. Hesse’s diagram with my more elaborate 
drawing. 
An accurate study of a single well-marked fan-system (hg. 449), on a scale of 
magnification three times as great, shows that it Starts from a complex of irregulär 
meshes, for the most part larger than those of the ensuing radiate portion. This initial 
complex ehanges rather suddenly into a radiating mesh-work with elongate meshes; 
as the septa spread out they fork, and the meshes also widen. Each System, then, 
repeats in little the System of an ordinary type such C. Waechteri, and the initial 
complex is as it were a repetition of the axial complex. In these Echinoderm tissues, 
vvhere stereom is less stroma is more ; and the function of the ordinary axial complex 
is to convey nutrient fluid through lacunae in the stroma, and to facilitate the passage 
of nerves. In cases of fairly rapid fossilisation, whether of Echinoderms or of other 
invertebrate skeletons, the stroma does not all decompose but becomes carbonised, 
and those tissues or tracts of the stereom in which it was most abundant are 
therefore marked in such fossils by a darker colour. 1 This may explain the darker 
patches that mark the initial complexes, even though the darkening substance can 
not be resolved into grains of carbon, as it can in the radioles of C. trigona. 
1 For proof and examples of these Statements, see «Note on the colour of certain Cyathocrini» 
in Bather, 1893, «Crin. Gotland, I.». Svensk. Vet.-Akad. Handl. XXV, No. 2, p. 151. Compare the 
interesting paper by R. Bullen Newton, 1907, «Relics of coloration in Fossil Shells», Proc. Malac. Soc. 
London, VII, pp. 280—292, pl. xxiv. 
