130 
COMPARATIVE ZOOLOGY. 
of horny fibres, which is the sponge of commerce. Cor¬ 
al is the solid framework of certain Polyps. There 
are two kinds: one represented by the common white 
coral, which is a calcareous secretion within the body of 
Fig. 95.—1, Vertical Section, and, 2, Transverse Section, of a sclerodermic Corallite: 
a, mouth; b, tentacles; c, stomach; d, intermesenteric chamber; e, mesentery; 
/.septum; g, endoderm; h, epitheea; k , theca, or outer wall; m, columella; ti, 
short partitions; p, tabula, or transverse partition; r, sclerobase; s, ccenenchy- 
ma, or common substance connecting neighboring corallites; t, ectoderm; x, 
pali, or imperfect partitions. 
the Polyp, in the form of a cylinder, with partitions ra¬ 
diating towards a centre ( scleroderm ); the other, rep re* 
sented by the solid red coral of jewelry, is a central axis 
deposited by a group of Polyps on the outside ( sclero¬ 
base ). The first 
sort is a dermal, the 
latter an epider¬ 
mal, exoskeleton. 
The skeleton of 
the Star-fish is a 
leathery skin in 
which are embed¬ 
ded calcareous par¬ 
ticles and plates. 
The Sea-urchin is 
covered with an 
Fig. 96.—Shell of Sea-urchin ( Cidaru ) without its spines. . n ., , , 
inflexible shell of 
elaborate and beautiful construction. The shell is really a 
