ROOM II.] NATURAL HISTORY. 61 
on the middle of the back. Their jaws are complicated, 
triangular, inclosed, and formed for crushing the food. 
The shells of these animals are strengthened by columns 
of calcareous matter which are deposited in the cavi¬ 
ties between the folds of the internal organs. Some 
species are convex, as Echinanthus, but they are mostly 
very flat and depressed, as the Scutella . Many of the 
latter are pierced with holes in the disk, or are lobed on 
the edge. 
The family of Galeritidce (Case 9) resemble the former 
family in the position of the mouth and vent, and in being 
covered with very small spines, but the lines of pores ex¬ 
tend from the middle of the back to the mouth ; they are 
Sometimes interrupted at the margin, but then they are to 
be seen again forming five distinct petal-like impressions 
on the under side near the edge of the mouth. The cavity 
of the shell is simple, and their jaws are small or only 
rudimentary. Only a few kinds are now found in a recent 
state, but the extinct species are very numerous, forming 
several very distinct genera. The vent in some of these 
is above the hinder margin, but it is never in the middle 
of the back. 
The family of Spatangidce (Cases 9, 10,) differ from all 
the others in the mouth being transverse, and more or 
less in front of the middle of the under side of the shell. 
They are destitute of jaws ; the series of pores are ge¬ 
nerally interrupted, but they form petal-like rows, both 
on the back and near the mouth. The shell is generally 
heart-shaped, with a groove from the crown to the mouth 
in front, and they are sometimes furnished with long 
slender as well as the usual small short spines. The 
cases or shells of these animals are formed of the same 
number of bands as the other Sea Eggs, but the bands are 
of unequal breadth, as is sometimes the case with deformed 
specimens of the hemispherical Echini . (See specimen in 
Case 2.) 
The Holothurida or Sea Cucumbers have many 
characters in common with the Echinidce , but their skin is 
only hardened with irregular scale-like or oblong bones; 
their mouth is in general surrounded with 5 or 10 plu¬ 
mose appendages. Most of them are furnished with 
tows of pores similar to, and for the same purpose as* 
