ALCYONARIA. 



221 



a rule. The septa are thus seen to be pseudo-septa, and 

 they cannot be regarded as being homologous with the septa 

 of the Zoantharia sclerodermata. The so-called ccenenchyrual 

 tubes are occupied by sacs lined by the endoderm, which 

 are closed externally, but communicate freely with the body- 

 cavities of the polypes by means of transverse canals ; and 

 Mr Moseley suggests, with great probability, that these are 

 really of the nature of rudimentary sexless polypes. 



The genus Heliopora is not known as occurring in the 

 fossil condition, but it is represented by various extinct 

 types, dating from the Lower Silurian period. 



Of the extinct types, the Silurian and Devonian genus 

 Heliolites (fig. 116) is the most important. It has a well- 



Fig. 116. A, Small colony of Heliolites megastomc, of the natural size ; B, Small portion of 

 the surface of the same, magnified, showing the calices (a) and the mouths of the ccenen- 

 chymal tubes (/>) ; c, Vertical section of the same, enlarged, showing the tabulate corallites 

 (a), and the tabulate tubes of the coeuenchyma (ft). (Original.) 



developed sclerodermic corallum, with comparatively large- 

 sized, tubular, regularly tabulate corallites, usually possess- 

 ing distinct but rudimentary septa, intermingled with a 

 copious ccenenchyma formed of tabulate geometric tubuli, 

 much smaller than the corallites, and destitute of septa. 

 The so-called " coenenchymal " tubules were probably occu- 

 pied in the living state by rudimentary or imperfect polypes. 



