Structure 0/ Heteropora neozelanica, Busk. 331 



have not been as yet satisfactorily established, though they 

 have been usually regarded as serving in some way to place 

 the cavities of the polypides in direct communication*. With 

 regard to the internal structure of the genus, the existence of 

 cross partitions or "tabulae" in the tubes was long ago 

 pointed out by Jules Haime, as regards his H. conifera and 

 H. pustulosa (Mem. de la Soc. Geol. de France, vol. v. p. 208, 

 1854). Mr. Busk ('Crag Polyzoa,' p. 122) pointed out that 

 the cancelli enter not at all or rarely into the central axis of 

 the branches of the skeleton, this being made up of the thin- 

 walled and polygonal proper zocecia. The same observer 

 also pointed out that the " ostioles," or apertures of the can- 

 celli, are often " completely closed by a calcareous depressed 

 lid, which in the majority of cases, however, is perforated in 

 the middle; " and he expressed the belief that "the remains 

 of these hymen-like lids," left behind at successive stages of 

 growth, might probably account for the existence in the inter- 

 stitial tubes of some species of " partial transverse, nearly 

 equidistant septa," giving to the tubes in question a " pecu- 

 liar moniliform aspect." Mr. Busk further indicated that in 

 one species of the genus (viz. H. clavata of the Crag) " the 

 interstitial orifices, or many of them, exhibit a stellate ap- 

 pearance, owing to the projection into their interior of nume- 

 rous minute rays; affording thus another curious, false resem- 

 blance to a true coral." With this exception, nothing which 

 could be compared with the " septa "f of the Ccelenterata has 



* As the difference between the cancelli and the proper zocecia is one 

 of size and shape merely, and as both sets of tubes are precisely alike in 

 their internal structure, it may be regarded as tolerably certain that the 

 former were occupied by a set of zooids essentially similar to those inha- 

 biting the zocecia, but modified or specialized in some way. On this 

 view, the colony would be a truly dimorphic one. As for the perforated 

 calcareous or chitinous opercula covering the mouths of the cancelli in 

 parts of the skeleton (as described by Waters), we may suppose that 

 these do not exist to begin with, but that they are developed in the last 

 stages of the life of the zobid, and that they are produced successively 

 from below upwards as the area of active vitality is successively carried 

 further from the fixed base of the organism (as we see to be the case in 

 the coralla of various species of Favosites). 



f Professor Busk, in his descriptions of the species of Heteropora. 

 frequently employs the term 9 septa " to indicate the transverse plates 

 which intersect the tubes of certain forms of the genus. Mr. Waters has 

 followed Prof. Busk in this, or has sometimes employed the term " dis- 

 sepiments" for the same structures. It need hardly be pointed out that 

 these terms have such a totally different significance among the Ccelen- 

 terata, that their use in this connexion is undesirable, and is apt to lead 

 to confusion. The term " septa," in fact, should be in all cases confined 

 to the radiating and vertical elements of a calcareous skeleton ; and the 

 plates so named in Heteropora are the analogues of the " tabidce " of the 

 Ccelenterates. 



24* 



