STRUCTURE AND FUNCTION OF THE SPHAIRIDIA. 43 
The calcareous matter of the globule is a hard and ex- 
ceedingly brittle, vitreous, transparent carbonate of lime, 
deposited in smooth or rough (figs. 4 and 19), more or less 
concentric layers. What appears as calcareous matter in the 
fresh condition is not entirely such, as the true calcareous 
substance is deposited in or enclosed between layers of an 
organic stuff, as to the exact nature of which I am in doubt, 
though it is probably an albuminoid remnant of the cell walls 
which enclose the carbonate at the time it was deposited. 
This animal substance is most conspicuous between two ad- 
joining calcareous lamellz ; it appears in some cases in optical 
section of the globule in form of radiating threads, which are 
the bounding lines of elongated cones of the vitreous substance 
(figs. 2, 5, 7, and 9). In tangential section these cones are 
circular or polygonal in outline, are largest at the surface, and 
decrease in size towards the centre of the globule. The radiate 
cones of adjoining lamelle (truncate, of course, for each plate 
save the central one) may not exactly coincide, and there 
results the appearance shown in fig. 8. This hyaline carbonate 
does not differ from that composing the spines except in the 
manner of the arrangement of the parts, and even this dif- 
ference is only relative since nearly all the stages in the deve- 
lopment of the globule from the spine may be observed in the 
young Urchins (fig. 16 and 17). The structure of the neck of 
the globule is never very different from that of the spines, while 
the head is usually so greatly changed by the increase in thick- 
ness of the calcareous rods and cross bars and the disappear- 
ance of a part of the canals of the spine that its lamellate 
structure is with difficulty referred to its original type. The 
elongate forms are frequently rough surfaced, the inequalities 
consisting of knobs and blunt points, most numerous on the 
apex of the head, but frequently extending downwards to the 
neck. They are often perforated by a branch of the canalicular 
system or may be situated at the side of such an opening. 
Canal System.—If the spheridia, whose canal system it is 
desired to study, be placed in a small quantity of dilute acid! 
1 T used successively H,SO4, HNO*, HCl, chromic, acetic, osmic, and osmo- 
