FOSSIL ACTINOZOA. gi 



by a series of upright partitions or " septa," which spring from 

 the inner surface of the theca, and advance toward its centre. 

 Very commonly some of the septa unite centrally with a 

 median calcareous rod, which extends vertically from the 

 bottom of the theca to the bottom of the calice (sometimes 

 projecting into the latter), and which is called the " columella." 

 The columella, however, is often wanting, or a spurious one 

 may be formed by the twisting together or coalescence of the 

 inner edges of the septa. In rare cases, also, the septa them- 

 selves are wanting. The septa, further, are of different 

 breadths. A certain number (fig. 43) extend quite to the 





Fig. 43. Diagrammatic sections of corals. A, Section of sclerodermic coral, showing 

 five primary septa, the columella, and costae (c) ; B, Section of Rugose coral, showing 

 four primary septa. Between the primary septa are seen the secondary and tertiary 

 septa. 



centre of the coral, where they meet the columella (when this 

 is present). These are called the " primary septa." Others, 

 however, fall short of the columella by a greater or less dis- 

 tance ; and these are called "secondary" and "tertiary" 

 septa, according to their breadth. 



The above is the essential structure of the typical form of a 

 simple sclerodermic coral, and it is easy to see that it is pro- 

 duced by the calcification, or conversion into carbonate of 

 lime, of the lower portion of a polype similar in structure to 

 an ordinary Sea-anemone. The " theca" of the coral corre- 

 sponds to, and is secreted by, the " column-wall " or general 

 wall of the body of the polype. The " septa," again, corre- 

 spond with the " mesenteries," and, like them, are " primary," 

 " secondary," or " tertiary," according as they reach the centre 

 or fall short of it by a greater or less distance. We must re- 

 member, however, that it is only the inferior half of the body 

 of the polype which is thus calcified. The tentacular disc and 

 mouth are placed at some distance above the upper margin of 

 the theca, and the digestive sac occupies the calice; whilst 



