CCELENTERATA. 



the whole ot the space comprised within the theca is lined by 

 the endoderm, and the whole of its outer surface is covered 

 by the ectoderm. 



Whilst the above gives the fundamental structure of a simple 

 sclerodermic corallum, there are some ad- 

 ditional details which are of sufficient im- 

 portance to demand special notice. The 

 septa in the coralla of the Zoantharia 

 sderodermata, like the mesenteries of the 

 living animal, are in multiples of five or 

 six. It is not common, however, to find 

 the septa so few as would be represented 

 by these fundamental numbers. Com- 

 monly, in progress of growth, fresh septa 

 are formed between those originally pre- 

 sent, until there may be several " cycles " 

 of septa (fig. 43). 



The septa may be considered as being 

 continued, in many cases, through the 

 theca, and beyond its external surface. 

 The outer surface of the theca thus comes 

 to be covered with a series of vertical 

 ridges or ribs, which are termed the " cos- 

 tae" (fig. 43, c, and fig. 44). The costae 

 vary much as to the distance by which 

 they are separated from one another, and 

 as to their breadth, their solidity, and 

 their ornamentation with spines, tuber- 

 cles, or teeth. 



In some cases the outer surface of the 

 theca is more or less covered by a thicker 

 or thinner envelope of calcareous matter, 

 constituting what is termed the "epi- 

 theca." In some cases, as in the genus 

 Montlivaltia (fig. 45), the epitheca is very 

 highly developed, and forms a dense 

 covering, but it is often extremely thin. It varies much as 

 to the extent to which it is applied to the theca ; and it may 

 be smooth, or may be marked by concentric or encircling 

 ridges. 



The chief remaining structures which may be noticed are 

 what are called " pali," " dissepiments," and " tabulae." Pali 

 are "small processes which exist between certain septa and 

 the columella. They generally arise from the base of the vis- 

 ceral cavity, or close to it, and pass upwards, united by one 



Fig. 44. Tnrbinolia 

 rulcata. The upper figure 

 shows the exterior of the 

 theca with the costae. The 

 lower figure shows the ca- 

 lice, with the columella 

 and primary and second- 

 ary septa. Eocene. 



