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, epitheca; k, theca, or outer wall ; m, columella; n, 

 short partitions ; p, tabula, or transverse partition ; r, sclerobase ; *, 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 in 

 which are embed- 

 ded calcareous par- 

 ticles and plates. 

 The Sea-urchin is 

 covered with an 



FIG. 96. Shell of Sea-urchin (Cidaris) without its spines. j n fl ex ibl e shell of 



elaborate and beautiful construction. The shell is really a 



