CNIDARIA 99 



support, consists of roundish crystalline bodies, which sub- 

 sequently fuse with one another. The earliest fundaments 

 of the calcareous septa [sclerosepta] soon make their appear- 

 ance. It was shown by Milne-Edwards et Haime, and 

 afterwards by Lacaze-Ddthiers, that the calcareous septa 

 correspond in position each to a gastral pouch, and therefore 

 that they occur between every two mesenterial septa.. The 

 earliest fundaments of the twelve primary sclerosepta are 

 called radial ridges (Sternleisten), and at first are V" or 

 Y-shaped (Fig. 46). The fundament of the theca (Matier- 

 blatt) arises by the peripheral ends of the radial ridges soon 

 becoming fused with one another. All of these are struc- 

 tures which are secreted by the ectoderm of the pedal disc, 



Fig. 45. — Development of the calcareous skeleton of Ast)oides ealycalaris (afler 

 V. Koch), diagrammatic. The section is made perpendicular to the pedal dit-c 

 in the direction of a secant. At the bottom the fundament of the basal plate; to 

 the left the epitheca ; to the right two radial ridges [sclerosepta] growing upwards 

 from below, alternating with two mesenterial septa [sarcosepta]. 



and naturally the more these skeletal parts rise upwards the 

 more the ectodermal layer of the pedal disc must undergo a 

 kind of invagination. It follows from this that in later stages 

 also those parts of the skeleton which apparently lie inside 

 the body of the polyp are covered by an epithelial lamella 

 belonging to the ectoderm of the pedal disc (calycohlast laijer, 

 V. HbiiDBR). But the lateral walls of the body in its lower 

 portions also deposit externally a calcareous layer, which 

 constitutes the fundament of the so-called epitheca (Fig. 45). 

 The so-called columella is formed by the fusion of the radial 

 ridges [sclerosepta] with one another at their inner, central 

 ends. Six of the twelve radial ridges soon become more 

 prominent, so that there is established an arrangement in 

 two cycles. Subsequently other cycles make their appeai-- 



