130 



COMPARATIVE ZOOLOGY. 



of horny fibres, wliich 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 ; 6, tentacles ; c, storaacli ; d, iutertuesenteric chamber ; c, mesentery ; 

 /, septnm: 17, eiidoderm; A, epitheca; k, theca, or onter wall; Tti, columella; 7*, 

 short partitions ; p, tabula, or transverse partition ; r, sclerobase ; 8, coenenchy- 

 mii, or common substance connecting neighboring eorallites ; t, ectoderm ; ar, 

 piili, or imperfect partitions. 



the Polyp, in the form of a cylinder, with partitions ra- 

 diating towards a centre {scleroderiii) ; 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- 



hase). The first 

 ^|V?^B^^^^^^^te%^ sort is a dermal, the 



latter an epider- 

 mal, exoskeleton. 



The skeleton of 

 the Star-fish is a 

 leathery skin in 

 whicli are embed- 

 ded calcareous par- 

 ticles and plates. 

 The Sea-urchin is 

 covered with an 



Fia. 96.— Shell of Sea-urchiu (CWorw) without its spines. . n 



mnexible shell of 

 elaborate and beautiful constrnction. The shell is really a 



