338 



COMPARATIVE ZOOLOGY 



of earthy material within the body, and is nearly con- 

 fined to the vertebrates. The exoskeleton may be of 

 two kinds dermal and epidermal. 



Some of the Protozoa, as Radiolaria and Foraminif- 

 era, possess siliceous and calcareous shells of the most 

 beautiful patterns (Fig. 2). The toilet sponge has a 

 skeleton of horny fibers, which is the sponge of com- 

 merce. Coral is the solid framework of certain polyps. 

 There are two kinds : one represented by the common 



FIG. 292. i, Vertical Section, and, 2, Transverse Section, of a sclerodermic Corallite: 

 a, mouth; b, tentacles; c, stomach; d, intermesenteric chamber; e, mesentery; 

 _/", septum; g, endoderm; k, epitheca; k, theca, or outer wall; in, columella; , 

 short partitions; /, tabula, or transverse partition; r, sclerobase; s, ccenenchyma, 

 or common substance connecting neighboring corallites; /, ectoderm; x, pali, or im- 

 perfect partitions. 



white coral, which is a calcareous secretion within the 

 body of the polyp, in the form of a cylinder, with par- 

 titions radiating toward a center (scleroderm)\ the 

 other, represented by the solid red coral of jewelry, is 

 a central axis deposited by a group of polyps on the 

 outside (sclerobase}. 



The skeleton of the starfish is a leathery skin in 

 which are embedded calcareous particles and plates. 

 The sea urchin is covered with an inflexible shell of 

 elaborate and beautiful construction. The shell is really 

 a calcified skin, being a network of fibrous tissue and 



