130 
COMPARATIVE ZOOLOGY. 
skeleton of horny fibres, which is the sponge of commerce. 
Coral 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 , eudoderm; h, epitheca; k, theca, or outer wall; m, columella; «, 
short partitions; p, tabula, or transverse partition; r, sclerobase; a, 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, repre¬ 
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 stud¬ 
ded with calcare¬ 
ous particles and 
plates. The Sea- 
urchin is covered 
with an inflexible 
Fig. 96.— Shell of Sea-urchin {Cidaris) without, its spines. . 
shell oi elaborate 
and beautiful construction. The shell is really a calcified 
