128 COMPARATIVE ZOOLOGY. 
their shells. When the loss is periodical, it is termed 
moulting. 
2. The Skeletons.—(1) The Exoskeleton is developed 
by the hardening of the skin, and, with very few excep- 
tions, is the only kind of skeleton possessed by inverte- 
brate animals. The usual forms are coral, shells, crusts, 
scales, plates, hairs, and feathers. It is horny or calca- 
reous; while the endoskeleton is generally a deposit of 
earthy material within the body, and is nearly confined 
to the Vertebrates. 
The microscopic particles of living jelly, called Poly- 
cistines and Foraminifera, cover themselves with sili- 
ceous and calcareous shells of the most beautiful pat- 
terns. The Sponge has an internal skeleton of horny 
fibres, which is the sponge of commerce. Coral is the 
Fic. 95.—1, Vertical Section, and, 2, Transverse Section, of a Sclerodermic Corallite : 
a, mouth; b, tentacles; c, stomach; d, intermesenteric chamber; e, mesentery ; 
Ff, septum; g, endoderm; h, epitheca; &, theca, or outer wall; m, columella; n, 
short partitions; py, tabula, or transverse partitions, a characteristic of extinct 
corals; 7, sclerobase; s, cenenchyma, or common substance connecting a neigh- 
boring corallite ; ¢, ectoderm; 2, pali, or imperfect partitions. 
solid frame-work of certain Polyps. There are two kinds: 
one represented by the common white coral, which is a 
calcareous secretion within the body of the Polyp, in the 
form of a eylinder, with partitions radiating toward a 
centre (sclerodermic); the other, represented by the solid 
red coral of jewelry, is a central axis deposited by a group 
of Polyps on the outside (sclerobasic). 
