126 



AX IXTRODUCTIOX TO THE STUDY OF FOSSILS 



outlining the size and shape of the ba?e of the soft parts of the 

 animal. At the same time the soft basal portion between the 

 mesenteries bends upward, permitting a greater deposit of lime 



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Fig. 47. — Vertical section through the edge of a ver\' young stage of the coral, 

 Astroides calicularis, which has fixed itself to a piece of cork (r.)- Very much en- 

 larged. The deposits of lime {b., ep.) are apparently thrown down outside the 

 ectoderm {ec.) but through the agency of these cells, b., basal plate ; ec, ectoderm ; 

 en., endoderm ; ep., epitheca ; ing., mesogloea ; m., mesenter>' ; 5., septum, 

 developed by upward bending of wall of animal between the mesenteries (m.-m.) 

 upon the basal plate (b.). (Redrawn from Lankester's Zoology, after Von Koch.) 



here ; since the mesenteries radiate from the center to the edge 

 of the soft body portion so these ridges (septa) of calcium car- 

 bonate radiate likewise. These septal ridges grow higher and 

 higher, resulting finally in the thin septa, over each of which are 

 folded the three layers — ectoderm, mesogloea and endoderm. 

 The upper edges of the septa are conspicuously toothed, 

 giving them a saw-like appearance. The septa are seen to alter- 

 nate with the mesenteries but are external to the polyp, i.e. each 

 septum corresponds to a radial inpushing of the base and lower 

 sides of the polyp. The outer portion of the barrel-shaped 

 body of the animal early bends upward faster than the inner 

 portion ; it thus results that the non-porous deposit of lime 

 thrown down by the ectoderm takes on a cup-like appearance, 

 called the calyx, with the septa extending from the edge of the 

 cup down to its center. Later the longest septa, expanding, 

 unite with one another to form a spongy, central mass, the 

 pseudo-columella . 

 As the animal grows in circumference and adds new mesen- 



