BOOM X.] NATURAL HISTORY. 43 
In the Shield Echini the shell is very solid and thick, 
and supported, internally, by perpendicular columns, so 
that they are capable of resisting the action of the waves 
for a considerable time. The mouth is armed with jaws, 
inserted in five triangular spongy bones, and the pores are 
placed in five arched pairs of bands, forming a star on the 
upper part of the shells. Some of the species of this 
group, as the Clypeasters (Case 2), are convex and shield¬ 
shaped ; the others, as the Echinodisci and Scutellae (Cases 
2 and 3), are so flat and depressed, that it is difficult to 
conceive how the animal can exist in so narrow a cavity. 
Many of the species of this genus are pierced with holes 
through the disk, ( Scutella quinquifora, Scutella bifora ,) 
&c. Others are lobed on the margin, as the Eight-rayed 
Scutella ( Scutella octodactyla ) and Toothed Scutella 
(i Scutella dentata). 
In the third group, the bodies are orbicular, more or less 
depressed, with the two openings of the alimentary canal 
placed opposite each other in the axis of the shell, one at 
the vertex, the other at the base, and the series of pores 
forming bands, extending from the one to the other. These 
shells are generally covered with larger spines and tuber¬ 
cles. In many species of this division (Cases 3—6) the 
spines are of nearly equal size, and the tubercles on which 
they are placed not pitted in the centre ( Echini ) ; as the 
common Sea-Egg of the English coast ( Echinus esculentus ), 
much sought after as food during a part of the summer 
season, at which time the shell is almost entirely filled 
with eggs. Other species, in which the tubercles are of the 
same form, have some spines much longer than the rest 
(Echinometrce ) ; as the Spiniferous Sea-Egg ( Echinus lu- 
cunter ), the Triangular-spined Sea-Egg ( Echinus trigona- 
rius), and the Artichoke, or Black Sea-Egg ( Echinus 
atratus ), peculiar for the larger spines being very short 
and truncated, forming a smooth surface, somewhat re¬ 
sembling a tessellated pavement. In some species with 
spines of unequal size, the tubercles to which they are 
attached are pierced in the centre ( Cidaris ). Those called 
Turbans are of a spherical form, and have very narrow 
wavy rows of pores, as the Imperial Turban ( Cidaris im - 
perialis) and the Porcupine Turban ( Cidaris hystrix); 
while those which are depressed, and have narrow separate 
