228 



CXELENTEBATA. 



according to the density of this deposit, there is produced a leathery, 

 chalky, or even stony framework. 



If isolated needles or toothed rods (fig. 173) of calcareous substance 

 are distributed beneath the epidermis and the ccenenchyma, the 

 polyp-stock has a fleshy, leathery nature (Akyonaria) ; but if, on the 

 contrary, the calcareous structures are fused together or are cemented 

 together in a larger mass, a solid, more or less firm, often stony cal- 

 careous skeleton is developed (Madreporaria). In the individual 

 animals the formation of this sub-epidermic skeleton begins on the 



foot surface, and advances 

 thence in such a manner that 

 near the calcareous foot-plate 

 there is formed in the under 

 part of the polyp body a more 

 or less cup-shaped theca, from 

 which numerous perpendicular 

 plates, the septa, radiate in- 

 wards. In the cup-shaped cal- 

 careous framework of the 

 individual polyp, the structure 

 of the gastrovascular cavity is 

 repeated, with the exception 

 that the calcareous septa cor- 

 respond to the interspaces of 

 the mesenteries (fig 174). 

 The number of the septa in- 

 creases as does that of the 

 mesenteries and tentacles with 

 the age of the polyp according 

 to the same laws. At the 

 same time a great number 

 of systematically important 



modifications of the skeleton are effected by further differentiation. 

 A column-like, calcareous mass sometimes arises in the axis of the 

 cup (columella), and in its neighbourhood a circle of calcareous 

 rods (pull), which ore separate from the septa (fig. 175). There may 

 further be formed between the lateral surfaces of the septa, processes 

 of calcareous substance as interseptal rods or horizontal shelves 

 (dissepimenta) ; also on the outer side of the wall of the theca ribs 

 (costai) projecting beyond its external surface, and similar dissepi- 

 ments may be produced between these. 



Fl<5. 173. Calcareous bodies (Scle,-odermHtrS) of 

 Alcyonaria (after Kolliker). a, of Plcjcaurella. 

 I, of Gurgun'ta. c, of Alcyonium. 



