TYPE CCELENTBRA. 



107 



Frequently the ectoderm of the anthozoan polyps secretes 

 a, skeletal substance which may either be carbonate of lime, or 

 else an organic substance of a horny consistency. In the 

 corals the secretion takes the form of carbonate of lime and 

 forms a cup (Fig. 58, ajo) in which the polyp is seated, ridges 



(mp ['hs 



Fig. 58. — Diagkam of thb Steuctuee of a Coeal (after von Koch, from Lano). 



ap = exotheca. As = mesentery. 



f'P = basal plate. ss = septa. 



Calcareous skeleton, white; ectoderm, shaded; mesogloea, black; endoderm, 



dotted. 



(septa, ss), over which the soft tissues of the animal are 

 moulded, projecting up from the bottom of the cup. The 

 septa may be united by delicate tangential bars, synapticulce, 

 and from the bottom of the cup a somewhat cylindrical colu- 

 mdla may project, other upright rods, the pali, intervening be- 

 tween the free edges of the septa and the columella. The 

 upper portion of the body-wall of the polyp is reflected over 

 the rim of the calcareous cup and may produce ridges, costce, 

 on its outer surface corresponding in position with the septa, 

 and inasmuch as the cup is continually increasing in depth so 

 long as the polyp lives, and the polyp only occupies the upper 

 portion, the lower part may from time to time be separated 

 off by a transverse partition or dissepiment. 



In other forms, such as the Alcyonarians, however, the 

 skeleton is secreted only by the basal ectoderm and the colony 



