132 NATURAL HISTORY. [NEW BUILDING. 
gether by the skin, and have a kind of organization dur- 
ing the life of the animal, they may easily be separated from 
each other after death, and then appear likeseparate hones. 
This structure allows the animal to increase both the size 
and the number of the pieces that compose its protect¬ 
ing case as the body grows, and also to repair, by the 
deposition of fresh calcareous particles on the skin of the 
healed part, any injury which the animal may have re¬ 
ceived from external accidents during its life. (See such a 
specimen, Case 3.) 
They are all marine, and live on animal food. The 
free kinds move about with their mouths beneath, and the 
attached ones are affixed by their backs with their mouth 
above, to enable their limbs to bring the food within its 
reach. 
The First Class, Ditremata, have a distinct di¬ 
gestive canal, furnished with a mouth and vent, contain¬ 
ing the Echinidci and Holothurida. 
The Echinida 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 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 fall 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 globular kinds have the mouth and vent 
placed opposite one another, in the centre of the upper 
and lower surfaces, w ith the bands of pores (or ambulacra y 
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 jaw^s, furnished with five rather projecting 
