THE STRUCTURE OP XENIA HTCKSONT. 271 



section the coelenteron and its two canals appear J.-shaped. 

 The canal opens into the base of a neighbouring coelenteron. 

 These canals are probably the representatives of the original 

 stolon from which all the primary coelentera grew out. 



The well-developed longitudinal canals running parallel to 

 and between the coelentera of the polyps of Xenia remind 

 one of the coenenchymal tubes of Heliopora coerulea, 

 described by Moseley (1881) and by Bourne (1895). The 

 resemblance is more striking when we consider that in both 

 cases the longitudinal tubes are lined by endoderm, and are 

 connected near the upper surface of the colony with a network 

 of superficial endodermic canals. 



The differences between the canals of Xenia and the coenen- 

 chymal tubes of Heliopora are chiefly due to the fact that in 

 the latter only the outer portion of the coral is living, the 

 internal parts consisting of calcareous skeleton only, whereas 

 in Xenia the whole colony is penetrated by living cells. In 

 Heliopora, therefore, the coelentera of the polyps and all canals 

 running into the colony must terminate within about 2 mm. 

 of the surface, as this is tiie lowest limit of the living substance. 

 The coenenchymal tubes of Heliopora have an exactly similar 

 course to the longitudinal canals of Xenia, as they run parallel 

 to the coelentera from their point of origin from the superficial 

 canal system (just beneath the ectoderm covering the 

 free surface) to the base of the living portion of the colony, 

 where they and the coelentera terminate blindly. 



Moseley (1881) believed the coenenchymal tubes of Helio- 

 pora to be degenerate siphonozooids from which the mesen- 

 teries had disappeared, but Bourne (1895) has shown that 

 they cannot be so regarded. 



In order to find a parallel to these coenenchymal tubes of 

 Heliopora, Bourne thought it necessary to go outside the 

 Alcyonaria and compare the tubes with certain longitudinal 

 canals of Millepora, described by Moseley (1876). After 

 carefully studying the canal system of Xenia, it appears to me 

 that this course is not now necessary, as the comparisons insti- 

 tuted above between the coenenchymal tubes of Heliopora 



