468 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



the tentacles soft and free (Fig. 327). When the coral 



%eems to disappear, as in a cell, it is simply the with- 

 drawal of the soft up- 

 " per parts into the lower 



calcareous part or thg- 

 ca. In the dead coral 

 the organic matter dis- 

 appears and leaves the 

 cuplike theca showing 

 the radiated structure 

 of the animal (Fig. 328, 

 a and b). 

 Theca. — Only recently has the origin of the theca 



been well understood. It was formerly supposed that 



the coral limestone was deposited in 



the substance of the tissues, and there- 

 fore constituted a sort of endoskeleton. 



But now it is known to be a true exo- 



skeleton or an epidermal 



structure. The mode of 



formation seems to be as 



follows: (1) The basal 0t "' 



Fig. 328.— a, stony part of a single coral ; 

 6, section of same, showing structure. 



Fig. 329. — Basal plate or foot disk 

 of a larva of coral, showing the 

 commencing lamella; of the theca. 

 (After Sedgwick. ) 



Fig. 330.— Diagram showing struc- 

 ture of the theca ; the white in 

 section is coral limestone: Bp, 

 basal plate ; 77*, theca ; ep, epi- 

 theca ; ec, ectoderm or epiderm ; 

 en, endoderm ; T, tentacles ; ce, 

 oesophagus ; ot, mesentery ; mf, 

 mesenteric filaments. (After 

 Sedgwick. ) 



