l6o MANUAL OF ZOOLOGY. 



with the separate polypes of the colony. On the other hand, 

 the sclerodermic corallum (when not composed simply of 

 scattered spicules) either consists of a single cup-like structure 

 corresponding with a single polype (fig. 74), or of several such 

 ( n g- 73? A) united by a common skeleton. 



A typical simple sclerodermic corallum (fig. 74) is secreted 



Fig 74. Caryophyllin borealis. A simple sclerodermic Coral, twice the natural 

 size. (After Sir Wyville Thomson.) 



by a single polype, and its structure presents an obvious cor- 

 respondence with that of the animal which produces it. It is 

 generally more or less conical in shape, sometimes discoid, 

 consisting of an outer wall and included space. The wall is 

 secreted by the mesoderm of the column and base, and is 

 known as the " theca." It may be very imperfect, or may be 

 strengthened by a secondary calcareous investment ("epitheca.") 

 The theca encloses a space which corresponds with the lower 

 part of the body-cavity of the polype, and is known as the 

 " visceral chamber." Superiorly the theca terminates in a 

 shallower or deeper cup-shaped depression, which contains 

 the stomach-sac of the polype, and is known as the "calice." 

 Below' the calice, the visceral chamber is subdivided into a 

 number of vertical compartments (" loculi ") by a series of 

 upright partitions or " septa," which spring frojn- the inner 

 surface of """thlflliecaj and are directed inwards towards the 

 centre. The septa are calcifications formed within the me- 



