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; 6, tentacles; c, stomach; d, intermeseuteric chamber; e, mesentery; 

 /, septum: q, endoderm; h, epitheca; k, theca, or outer wall; m, columella; n, 

 short partitions; p, tabula, or transverse partition; r, sclerobase ; s, coenenchy- 

 ma, or common substance connecting neighboring coiallites ; t r 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 (Cidarits) without its spines. in i» i . 



shell oi elaborate 



and beautiful construction. The shell is really a calcified 



