71 
bury themselves in the sand when they are left room x. 
on the beach by the retreating tide; it is 
pierced with rows of minute pores, through 
which are emitted small tentacula with dilated 
ends, by which the animal is enabled to attach 
itself to rocks and other marine bodies. The 
Sea-Eggs have been divided into several groupes, 
according to the shape of the body, and the po¬ 
sition of the mouth and vent. 
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 vent 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 is destitute of teeth, 
and often surrounded with a series of tenta¬ 
cula. 
In the second group, the mouth is in the 
centre of the under part, and the vent placed 
in or under the hinder margin. In some of 
these the shells are thin, and the series of pores 
are 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 per¬ 
pendicular columns, so that these shells resist 
the action of the waves for a considerable time. 
The 
