52 
thickly scattered over the shell in the intervals between the 
primaries on the peristomal and anal membranes, and on the 
ambulacral areas. ‘They converge around the bases of the 
primary spines and over the ambulacra, apparently for the 
protection of the more delicate structures found there. ‘They 
articulate by a simple ball-and-socket joint, without a central 
ligament. 
The minute structure of the primary spines is peculiar to 
all existing members of the family. These spines may be 
described as of terminate growth; that is, having arrived at 
a certain size, an end is put to the growth, and probably also 
to the life, of their upper portion, by a dense crust; this 
crust commences abruptly a short distance above the collar, 
into which the muscle which moves the spine is inserted, 
and extends over all the upper parts of the spine. ‘The crust 
is extremely liable to parasitic incrustations, and in no part 
of the spine so invested is there any evidence of repair after 
injury: this renders it probable that the soft animal mem- 
brane (perisoma), which invests the shells of all echinoderms, 
dies after the formation of this dense layer, as no other 
echinoderms are subject to similar attacks. 
On examining a transverse section of a primary spine, 
three layers may be recognised (Plate IV, fig. 1)—a central 
pith, formed of a loose sponge-like reticulation of calcareous 
fibres, which take in the main a longitudinal direction (this 
is seen better in longitudinal sections). Outside this pith is 
found a layer, which constitutes the greater part of the spine; 
it is composed of radiating plates, extremely thin where they 
arise from the pith, and gradually increasing in thickness as 
they approach the external crust, immediately below which 
the spaces between them are about equal to their own thick- 
ness; they are regularly and closely perforated, the holes 
being arranged.in rows, which are oblique to the direction of 
the spine. ‘These plates, when closely approximated, are 
simply united by bars, but if further apart, the bars branch 
and form the usual spongy network, which is so charac- 
teristic of the echinoderm skeleton. The external crust, 
although very dense, is perforated by minute tubes, which 
arise from the intervals between the plates of the middle 
layer, and terminate upon the free surface of the spine. The 
thickening of the crust forms the spinules, ridges, &c., which 
are so constantly found on the spines of the Cidaride. When 
these ridges are much marked, as frequently occurs near the 
tips of the spines, the plates of the middle layer commonly 
do not radiate from the pith to the crust, but certain of them 
conyerge to plates which retain their normal position, the 
