THE ANATOMY OF THE MADREPORARIA. 257 



The mesenteries vary in number, and are, like the entosepta, 

 generally of three orders. They are divisible into " pairs," as in the 

 other forms described, and possess the same arrangement of longit u= 

 dinal retractor muscles on their entocoelic faces, with the usual 

 difference in the two directive pairs. The trend of these muscles is 

 roughly indicated in fig. 1 3 ; but their minuteness renders it impos- 

 sible to recognise the arrangement of the protractor muscles, though 

 they are just visible in microscopic sections. There appears to be but 

 little contortion of the free edge of the mesentery, and the traces of 

 any organs resembling acontia are rare. This, however, may be due 

 to deficiency of material, which has much hampered my investigation 

 of this form. 



Both primary and secondary mesenteries appear to be united to the 

 stomatodceum for its whole length ; those of the third order become 

 disconnected from it very high up, and do not run deep down into 

 the colony, the cavities in which they lie disappearing among the 

 other perforations of the theca. 



The number of pairs of mesenteries right and left of the "directives" 

 is not necessarily equal. Complete systems both of mesenteries and 

 septa (1, 3, 2, 3, 1, in notation) are generally found only at the ends 

 of the long axis of the calyx, i.e. in the neighbourhood of the direc- 

 tives. This has been noticed in many other corals. 



That the almost exact correspondence of costse with septa, and of 

 the external lamellae (M' in the figures) with the mesenteries, adds to 

 the probability of the. correctness of v, Koch's view is undeniable. But 

 it is to be noted that no muscles are to be recognised on the mesoderm 

 plates of these lamella?, as would probably be the case had they once 

 been part of the mesenteries ; nor in the highest section of the decal- 

 cified polyp are any cases of decaying tissue visible where the growing 

 theca is supposed to have cut them. 



iii. Histology — This is of such a simple character as to hardly 

 require comment. The ectoderm is composed of simple columnar 

 cells, the endoderm of similar but more cubical cells. Calycoblasts 

 are present, but in small numbers in comparison with Flabellum. 

 Nematocysts are of two forms and sizes, of which, as in Flabellum, 

 the smaller is the only one occurring on the tentacles. Of the mesen- 

 terial filament, as unusual in outline, a sketch is given in fig. 1 6. 



In conclusion, I Lave to acknowledge my obligations to Professor 

 Moseley for much kind assistance and most of my material ; to Pro- 



