57 Be THE MEDALS OF CREATION. Cnap. IX. 
The common Echinus of our sea-coasts (chinus sphera), 
known by the name of Sea-egg, Sea-urchin, or Sea-hedgehog, 
presents the typical characters of this order of Radiata, 
which differs from the Crinoids and Star-fishes in the absence 
of arms. 
The calcareous envelope of the Echinus, or shell, as it is 
popularly called, is composed of many calcareous, polygonal 
plates, closely fitted to each other, and arranged in regular 
and elegant patterns ; appearing in the globular and spheri- 
cal kinds like the lines of the meridian on a globe. The 
plates are disposed in ten vertical series, united by ser- 
rated sutures, and form sections, into which the envelope or 
case very commonly separates upon the decay of the invest- 
ing integuments. Of the ten bands five are large and five 
small. The large bands (are) are each composed of a double 
row of plates, ornamented with tubercles supporting large 
spines (Lign. 100). The five narrower bands consist of much 
smaller plates, and the spines are minute, or altogether want- 
ing; hence these bands appear like avenues through the 
spinous tracks, and have therefore been fancifully termed 
ambulacra or walks. They are the equivalents of the grooves 
or furrows of the Star-fishes (Zign. 108, a.) ; and are tra- 
versed by numerous pores, for the exsertion of tubular feet, 
or tentacula. Besides these rows of minute openings, there 
are two principal apertures, the mouth and the outlet or 
vent ; and also a few large pores, commonly five, on the 
summit, for the exclusion of the ova, and the free admission 
of water. 
The form and relative position of the parts above de- 
scribed, afford characters by which the order is divided into 
genera. The mouth, which is situated on the lower part, is 
in some species furnished with five sharp angular teeth, at- 
tached to a calcareous frame-work that admits of being pro- 
truded ; this apparatus, when entire, commonly bears the 
name of “ Diogenes’ lantern.” The eminences on the surface 
