74 THE GENUS MONTICULIPORA. 



surface. In both, therefore, there are estabHshed two distinct 

 regions of the colony, an axial and a peripheral region. In 

 both, moreover, these two regions are very different in internal 

 structure, the tubes in the axial region of their course being 

 thin-walled and polygonal, while in the peripheral region their 

 walls are thickened, and they often become more or less 

 rounded in form. In both, further, it would appear that any 

 special interstitial tubes that may be present are developed 

 in the peripheral region only, and extend either not at all, 

 or to a very limited extent, into the axial part of the corallum. 



(2) As regards the dinwrpJiisiii of the corallum, all the 

 most characteristic and typical species of Montiadipora consist 

 of at least two, and sometimes of three, distinct sets of tubes, 

 which generally differ both in size and in internal structure, 

 and which are differently arranged in different cases. In 

 Heteropoi'a the skeleton consists of a series of large tubes 

 surrounded by smaller interstitial tubes ; but it does not appear 

 that there is any special difference in the internal structiwe of 

 these. In neither genus are we acquainted with the soft parts ; 

 and therefore we cannot assert positively that this dimorphism 

 has precisely the same significance in the two genera, while 

 there are orrounds for thinking that the reverse is the case. 



(3) As regards the structure of the wall^ the visceral 

 cavities of the tubes of JMonticulipora appear to be always 

 closed, no traces of any pores or canals in the wall having yet 

 been clearly proved to exist. In the case of Heteropora, on 

 the other hand, the thickened walls of the tubes, in the peri- 

 pheral part of their course, are traversed by an exceedingly 

 well-developed series of transverse canaliculi, which open into 

 the cavities of the tubes by definite pores, and which thus 

 place the body-chambers of contiguous zooids in direct com- 

 munication. These " canaliculi " differ structurally from the 

 " mural pores " of the Favositida;; chiefly in being tubes with 

 definite walls and dilated extremities, instead of being mere 

 circumscribed deficiencies in the wall. 



(4) No radiating " septa," in the form either of spines or of 



