NATURAL HISTORY. 
115 
ROOM II.] 
The First Class, Ditremata, have a distinct digestive canal, fur¬ 
nished with a mouth and vent, containing the Echinidce and Holothu- 
rida. 
The Echinidje, or Sea Eggs (Cases Nos. 1 to 10). These are 
covered with a hard case, formed of 40 perpendicular bands of square 
or six-sided pieces, sunk in the substance of the skin, and furnished ex¬ 
ternally with numerous spines, affixed by muscles on hemispherical tu¬ 
bercles, which allow the spines to move in all directions, protecting the 
animals from their enemies and enabling them to bury themselves in 
the sand on the shore when they are left by the retiring tide. 
These spines easily fall off when the animal is dead, and the greater 
part of the specimens exhibited are destitute of them. Ten pairs of 
the forty bands of pieces of which the cases are formed, alternating with 
the ten other pairs, are pierced with minute double pores, through which 
are sent out small filaments with dilated ends, which enable the animals 
to anchor themselves to marine bodies. These animals have two se¬ 
parate openings to their digestive canal. 
The more globular kinds have the mouth and vent placed opposite 
one another, in the centre of the upper and lower surfaces, with the 
bands of pores (or ambulacra , as they are called, from their fancied re¬ 
semblance to the walks in a garden) extending in five pairs of lines from 
the one to the other; the mouth is armed with very complicated jaws, 
furnished with five rather projecting teeth. (See Case 3.) These 
shells are generally covered with large spines. 
The Diadems ( Cidarim:, Case 1) have the tubercles on which the 
spines are affixed pierced with a central pit. 
The Echinidce ( Cases 2 to 6) have, on the contrary, simple rounded 
tubercles. The spines of most of the species of this family are equal¬ 
sized, but in some, especially such as are of an oblong shape, as Colo - 
bocentruSy the spines are large and club-shaped, and in others, as He - 
terocentrus , they are very short and truncated, forming in the mass, a 
smooth surface somewhat resembling a tessellated pavement. These 
animals are much sought after as food during the latter part of the sum¬ 
mer, at which time they are almost filled with eggs. 
The other Echinidce have the vent placed on the side of the 
body. 
The family of Scutellidje ( Cases 7, 8) have the rounded mouth in 
or near the centre of the under side. Their shell is covered with very 
minute equal-sized spines, and the pores form arched series like the 
petals of a flower, on the middle of the back. Their jaws are compli¬ 
cated, 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 cavities between the folds of the internal or¬ 
gans. 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 Galeritidje ( 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 extend 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 thie 
under side near the edge of the mouth. The cavity of the shell is 
