THE NATURE OF CAUNOPORA. 123 



of certain " Caunoporce " and " Diaporce" are provided with septal spines. I have 

 already described and figured these structures (Fig. 17), and need only repeat here 

 that in their structure and general arrangement they show nothing which would 

 distinguish them from the corresponding septal spines of a Syringoporaor a Favosites. 

 The only existing Hydrozoa which have any structures which could be confounded 

 with the " septa " of the Actinozoa are the Stylasterids, in some of which the 

 dactylopores of each cyclo-system are separated by thin radiating partitions or 

 "pseudo-septa" (Moselcy). These structures, however, have no resemblance to 

 the rows of septal spinules just alluded to as occurring in the interior of the tubes 

 of certain of the " Caunoporce " and " Diaporce" It appears, therefore, to be quite 

 certain that in all those " Caunoporce " and " Diaporce " in which the tubes possess 

 septal spines, the tubes must be foreign to the Stromatoporoid in which they are 

 found, and must belong to some Actinozoon. Moreover, as those " Gaunoporce " 

 and " DiaporcB " in which the tubes have septal spines are in no other respect 

 distinguishable from those in which the tubes appear to be without such spines, 

 it seems hardly possible to evade the conclusion that in the latter also the " tubes " 

 are foreign structures. 



(c) Syringopora as the Commensal of Caunopora. — Admitting that the so-called 

 " Caunoporce " and " Diaporce " are the result of the commensalism of some Coral 

 and some Stromatoporoid, the nature of the Corals concerned in the process still 

 remains for determination. The settlement of this point has proved a matter of 

 extreme difficulty, since the choice seems in most cases to lie between Syringopora 

 and Aulopora, and neither of these genera fulfils all the requirements of the case. 



If we take the larger and more massive examples of " Caunopora," and imagine 

 the investing Stromatoporoid to be removed, there is no doubt but that the 

 aggregate of the embedded tubes would show a close general resemblance to the 

 corallum of Syringopora. In such examples the " Caunopora-tubes " are very long, 

 run parallel with one another, and are connected by cross-branches which some- 

 times give origin to new vertical tubes instead of opening into an adjoining tube. In 

 their internal structure, also, the tubes would answer very well for Syringopora-tubes. 

 The tabulae of Syringopora, though usually funnel-shaped, are sometimes simply 

 flat or curved (e. g. in S. geniculata, Phill.),and sections of the " Caunopora-tubes " 

 show all the phenomena which are seen in similar sections of Syringopora, as regards 

 the " tabulae." The septal spines of the " Caunopora-tubes" are likewise — when 

 present — quite like those of Syringopora, except that there appear to be only eight 

 rows of these structures in each tube, whereas there are generally from twelve to 

 twenty of such rows in Syringopora. 



There are, however, in spite of these resemblances, great difficulties in the way 

 of supposing that the " Caunopora-tubes " are really referable to Syringopora. In 

 the first place, very many " Caunoporce" and most "Diaporce" are not massive, 



