64 THE GENUS MONTICULirORA. 



The essential character of the genus is thus the possession 

 of a skeleton made up of tivo kinds of tubes, larger and smaller, 

 the latter being the most numerous. The former have always 

 been regarded as the proper zocecia ; but the relations of the 

 interstitial tubes or '' cancelW" to the rest of the organism 

 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. conifcra 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 zooecia. 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 numer- 



^ As the difference between the cancelli and the proper zooecia is one of size and 

 shape merely, and as both sets of tubes are precisely alike in their internal struc- 

 ture, it may be regarded as tolerably certain that the former were occupied by a set 

 of zooids essentially similar to those inhabiting the zooecia, but modified or special- 

 ised 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 can- 

 celli 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 zooid, 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 oi Favosiics). 



