218 CEYLON PEARL OYSTER REPORT. 



I have entered at some length into this question of secondary deposit or 

 " stereoplasm," as there appears to be unquestionable evidence of its existence in 

 Heterocyathus, and the minute structure of the corallum of this genus may prove 

 serviceable in the interpretation of the structure of some fossil corals. Miss Ogilvie 

 (41, pp. 93-99) denies the existence of a "stereoplasm" in both recent and fossil 

 corals, and she was perfectly right as regards the corals that she describes. There 

 is, however, a close analogy between the secondary thickening or stereoplasm described 

 by von Koch (29) in Pholidophyllum and that of Heterocyaihus, and it is possible 

 that the structure of Lonsdaleia indica, as described and figured by Waagen (51), 

 may be referred to the same type, but in neither case are the authors' figures suffi- 

 ciently detailed to enable one to speak with certainty. The description given by 

 Frech (19) of Idastrcea profunda, and quoted at length by Miss Ogilvie (41, p. 99), 

 might be applied with very little correction to Heterocyathus, but in this case again 

 the author's figure is on too small a scale to enable one to judge whether the structure 

 is identical or not with that here described. 



General Anatomy of the Polyp. As described by Gardiner (23), 

 the corallum is completely invested by the tissues of the polyp, the latter being 

 interrupted only at the mouth of the Aspidosiphon chamber. It must be borne in 

 mind that the young Heterocyathus is attached to and grows round a gastropod shell 

 (usually a Cerithium) tenanted by an Aspidosip>hon, and that the growth of the coral 

 and the Sipunculid proceed pari passu till we get the intimate association between 

 the two forms characteristic of their adult condition. The shell, which served for the 

 original habitation of the Sipunculid and the surface of attachment for the coral, is 

 completely overgrown and eventually greatly exceeded in size by the latter. It is 

 evident, then, that as the coral grows, its soft tissues must be folded down all round 

 the edge of the cup to form a " perisarc," using that term in the restricted sense 

 given to it by Bernard (4, p. 21). As the corallum grew round the shell, the 

 perisarc must have kept pace with it ; indeed, it would be more correct to say that 

 the soft tissues enveloped the shell, and that their innermost ectodermic or calicoblastic 

 layer secreted the corallum which eventually enclosed the shell, and, growing beyond 

 its limits, formed the Aspidosiphon chamber. Eventually the edges of the perisarc, 

 growing round the shell on all sides, meet and unite below, excepting in the region of 

 the mouth of the shell. Thus we get a basal union of the perisarc similar to that 

 observed in Fungia, and as in the last-named genus we find that the mesenteries are 

 prolonged into the cavity of the perisarc and divide up the extra-thecal ccelenteron 

 into as many entoccelic and exocoelic chambers as there are mesenteries. The 

 mesenteries, however, do not extend to the centre of the base of the coral. They may 

 be traced as far as the costse, but die out where the costre pass into the central 

 irregular basal granulations, and here the soft tissues appear to be supported on 

 the granulations in the manner described by Fowler (16). Above the level of 

 the synapticula, which not only physiologically replace but are in some respects 



