ZOANTHARIA — MADREPORARIA 387 



the inner wall of the cup between the septa are called the 

 " dissepiments." Corresponding with the septa there may be a 

 circle of columns or bands rising from the basal parts of the 

 prototheca — the "pali"; and from the actual centre a single 

 column called the " columella." The longitudinal ridges on the 

 outside of the theca, corresponding in position with the septa 

 inside, are called the "costae" (Fig. 167, E, c). 



We may imagine that in the primitive forms that gave rise 

 to colonies, the episarc of the primary zooid overflowed on to 

 the substance to which it was attached, and gave rise to 

 successive layers of epithecal skeleton, which may be called the 

 " coenosteum." The ectoderm at the base of the original prototheca 

 is in some corals periodically dragged away from the skeleton, 

 and forms another cup or platform of lime at a little distance 

 from it — the "tabula." New zooids are developed at some 

 distance from the primary one by a process of gemmation in the 

 episarc, and independent thecae, septa, etc., are formed in it ; 

 the skeleton of the new zooid thus originated being connected 

 with that of the primary zooid by the coenosteum. 



There are many modifications of this simple description of 

 skeleton formation to be considered before a thorough knowledge 

 of coral structure can be understood, but sufficient has been said 

 to explain the use of the terms that it is necessary to employ in 

 the description of the families. "When it is necessary to speak 

 of the cup in which the zooid is situated without expressing an 

 opinion as to the homology of its wall, it is called the calyx. 



There are many forms of asexual reproduction observed in 

 the Madreporaria. Of these the most frequent is gemmation. 

 The buds are formed either on the episarc or on the canals 

 running between zooids at the surface of the coenenchym. When 

 the young zooids that have been formed by gemmation reach 

 maturity they have the same characters as their parents. Fission 

 occurs in the production of a great many colonies of Madreporaria. 

 It occurs occasionally in such genera as Madrepora and Pontes, 

 where reproduction by gemmation prevails, but it is said that 

 gemmation never occurs in those forms such as the Astraeidae 

 Fissiparantes where fission is the rule. In fission a division of 

 the zooid takes place in a vertical plane passing through the 

 stomodaeum and dividing the zooid into two equal parts. In 

 some cases these two parts become separated during the further 



