ROOM II.] NATURAL HISTORY. 59 
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 externally 
with numerous spines, affixed by muscles on hemisphe¬ 
rical tubercles, which allow the spines to move in all di¬ 
rections, 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 fail off when the animal is dead, 
and the greater part of the specimens exhibited are desti¬ 
tute of them. Ten pairs of the 40 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 separate openings to their diges¬ 
tive canal. 
The more spherical 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 resemblance 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 ( Cidaridce ), Case 1, have 
the tubercles on which the spines are affixed pierced with 
a central pit, and 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 
Echinometra lucunter , the spines are large and club- 
shaped, and in others, as E. atrata y 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 summer, at which time they are almost filled 
with eggs. 
The other Echinida have the vent placed on the side 
of the body. 
The Scutellidce (Cases 7, 8) have the rounded mouth 
in or near the centre of the under side. Their shell is 
