THE CARBONIFEROUS PERIOD. 177 
these grooves there seems to have been attached a row cf 
short jointed calcareous filaments or “ pinnules.” 
A few Star-fishes and Brittle-stars are known to occur in the 
Carboniferous rocks ; but the only other Echinoderms of this 
period which need be noticed are the Sea-urchins (Zchinozds). 
Detached plates and spines of these are far from rare in the 
Carboniferous deposits ; but anything like perfect specimens 
are exceedingly scarce. ‘The Carboniferous Sea-urchins agree 
with those of the present day in having the body enclosed in 
a shell, formed by an enormous number of calcareous plates 
articulated together. ‘The shell may be regarded as, typically, 
nearly spherical in shape, with the mouth in the centre of the 
base, and the excretory opening or vent at its summit. In both 
the ancient forms and the recent ones, the plates of the shell 
, 
Fig. 119.—Palechinus ellipticus, one of the Carboniferous Sea-urchins. The left- 
hand figure shows one of the ‘‘ambulacral areas” enlarged, exhibiting the perforated 
plates. The right-hand figure exhibits a single plate from one of the ‘‘inter-ambulacral 
areas.” (After M‘Coy.) 
are arranged in ten zones which generally radiate from the 
summit to the centre of the base. In five of these zones— 
termed the “ambulacral areas”—the plates are perforated by 
minute apertures or ‘ pores,” through which the animal can 
protrude the little water-tubes (‘‘ tube-feet””) by which its loco- 
motion is carried on. In the other five zones—the so-called 
‘“‘inter-ambulacral areas” —the plates are of larger size, and 
are not perforated by any apertures. In all the modern Sea- 
urchins each of these ten zones, whether perforate or imper- 
forate, is composed of two rows of plates; and there are thus 
twenty rows of plates in all. In the Palaeozoic Sea-urchins, on 
the other hand, the ‘‘ambulacrdl areas” are often like those 
of recent forms, in consisting of ¢zo rows of perforated plates 
(fig. 119); but the “inter-ambulacral areas’ are always quite 
M 
