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; &, tentacles; c, stomach; d, intermesenteric chamber; e, mesentery; 

 /, septnm ; g, endoderm ; h, epitheca ; k, theca, or outer wall ; m, columella ; n t 

 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, repre- 

 sented by the solid red coral of jewelry, is a central axis 

 deposited by a group of Polyps on the outside (sdero- 



lase). 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. , ,, /. , , 



and beautiful construction. The shell is really a calcified 



