58 NATURAL HISTORY. [n. ZOOL. GAL, 
the cellular substance of others, is often so filled with 
calcareous matter, as to leave, when the animal is dead or 
removed, a hard case or a stony coral; the latter often 
representing, in a very perfect manner, all the more import¬ 
ant characters of the animal. It is the hard parts or 
skeletons, as they may be called, of these animals, which 
alone can be shewn with any effect in collections, but 
whenever it is possible they should be studied in connec¬ 
tion with the animals which form them, as the animal 
alone affords the proper characters for their classification, 
and the study of the animal and coral together, enables 
the student to understand, in other cases, by the exa¬ 
mination of the coral alone, what was the probable 
structure of the animal that formed it. 
The Table Cases oe the Second Room contain the 
hard parts of the Echinodermata, so called from their 
body being covered with a hard coat formed of variously 
shaped calcareous pieces imbedded in the surface of the 
skin. These pieces are formed by the earthy particles 
being deposited round certain definite spots in the 
skin, and as they are developed, they assume a definite 
arrangement into certain distinct shapes peculiar to the 
different kinds ; although these are strongly united to¬ 
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 bones. 
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 Echinida and Holothurida. 
The Echinida or Sea Eggs; (Cases Nos. 1 to 10;) 
