122 BRITISH STROMATOPOROIDS. 



to the " Caunopora-tubes," tlie walls of which appear to be composed only of the 

 ordinary tissue of the Stromatoporoid-colony. Little stress, however, can be laid 

 upon this observation, as it might merely be a case in which the original walls of 

 the "Caunopora-tubes" had been gradually absorbed and "replaced" by the 

 Stromatoporoid, after the fashion so well known in the recent Hydradinia. 



3. It has hitherto proved impossible to demonstrate in a satisfactory way the 

 existence of any communication between the cavities of the " Caunopora-tubes " 

 and the mterlaminar spaces or zooidal tubes of the investing Stromatoporoid. 

 Some thin sections appear to show the occasional existence of such a communica- 

 tion ; others show no traces of anything of the kind. In the absence of any clear 

 and positive proof of the existence of such a communication we are precluded from 

 any comparison between the " tubes " of " Caunopora " and the gastropores of the 

 Milleporidce and Stylasteridce. The absence, therefore, of a proved coenosarcal 

 connection between the " Caunopora-tubes " and the investing Stromatoporoid is 

 in my opinion^ the strongest of all arguments in favour of the theory of commen- 

 salism — in spite of the great difficulties which this theory has to overcome. Indeed, 

 till such a connection can be shown to exist — and I am not prepared to assert 

 positively that it may not yet be shown to exist — it does not seem to me possible 

 to definitely accept any theory which would regard the " Caunopora-tubes " as 

 constitutent parts of the organisms in which they are found. 



4. Most well-preserved specimens of " Caunopora" and " Diapora" when 

 examined in thin sections, can be shown to have their tubes intersected by a larger 

 or smaller number of " tabulce," which are sometimes flat or simply curved, some- 

 times vesicular, and often infundibuliform. Very commonly the same tube will 

 be provided in part with flat tabulae, and in part with vesicular or funnel-shaped 

 tabulae (Plate X, figs. 1 and 2). I entertain little doubt but that the tubes of 

 " Caunopora " and " Diapora " really always possess these tabulae ; though owing 

 to imperfect preservation (as, for example, in most of the Devonshire specimens) 

 their presence may be difficult to demonstrate or they cannot be shown to exist at 

 all. Owing to the presence of these tabulae, longitudinal and transverse sections 

 of the tubes of "Caunopora " and " Diapora " possess a striking resemblance to 

 corresponding sections of the corallum of Syringopora, or, to a loss degree, of 

 Aulopora. It appears to me, however, that it is easy to give a far more than due 

 weight to this resemblance, since precisely similar "tabulae," exhibiting precisely 

 similar variations in their form and arrangement, can be shown to exist in the 

 astrorhizal canals of certain Stromatoporellce and in the axial canals of Idiostroma, 

 Stachyodes, and AmpMpora ; and it cannot be doubted that these tubes belong to the 

 Stromatoporoid in which they are found. 



5. A much more weighty argument in favour of the theory of commensalism 

 may be based upon the discovery, which I have recently made, that the " tubes " 



