5S 
enabled to attach itself to rocks and other marine bodies. 
The Sea-Eggs are divided into several groups, accord¬ 
ing to the shape of the body, and the position of the 
mouth and anal orifice. 
In the first group, Spatangus, the shell is nearly 
heart-shaped, with an oval compressed mouth placed in 
the front part of it on the under side, and the other ori¬ 
fice on the hinder margin. The upper part of the shell 
has the pores placed in five short bands, resembling a 
flower. The shells of this group are thin and brittle, 
and the mouth of the animal, destitute of teeth, is 
often surrounded by a series of tentacula. 
In the second group, the mouth is in the centre of the 
under part, and the other orifice placed in or beneath 
the hinder margin. In some of these the shells are 
thin, and the series of pores arranged in vertical bands, 
extending from the top of the shell to the mouth ; as in 
the genera Echinolampas and Galeries. 
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 are convex and shield¬ 
shaped ; the others, as the Scutellae, are very much de¬ 
pressed, and nearly flat, so that it is difficult to con¬ 
ceive how the animal can exist in so thin a cavity. 
Many of the species of this genus are pierced with 
holes through the disk, ( Scutella quinquifora , Scut, 
bifora), &c. Others are lobed on the margin, as the 
Eight-rayed Scutella {Scut, octodaciyla) and Tooth¬ 
ed Scutella {Scut, dentata ). In the third group, the bo¬ 
dies 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 
ROOM X 
Nat. Hist 
