STEUCTUEB AND EELATIONS OF CEETAIN COEALS. 
107 
into eight radially disposed chambers. The mesenteries consist of a median plate of 
homogeneous connective-tissue, which is directly continuous with the similar layer of 
the lining membrane of the calicular cavity, and also with that surrounding the 
stomach, and of an investment of entodermic cells covering the median plate on both 
sides, excepting where the retractor muscles intervene between the two. These retractor 
muscles form the lower borders of the mesenteries ; they consist of long stout fibres 
which, lying on the surface of the mesenteries, take origin from the lower part of the 
sides of the polyp-cavity, reaching down sometimes as far as the margin of the tabula, 
and curve inwards and upwards, becoming gradually more concentrated as they ascend 
to be inserted round the mouth and region just below it, in the intervals between the 
bases of the tentacles. 
The muscles have in position, with regard to the plates of the mesenteries, the same 
arrangement which Kolliker has described as existing in the Pennatulidse, and which 
has also been found in the genus Umbellula by Lindahl and figured (“ Om Penna- 
tulidslagtet Umbellula ,” till Kongl. Yet. Akad. inlemnad den 10. Feb. 1874: Stock- 
holm, tab. 1. fig. 8)*. 
The arrangement of the muscles is seen in Plate 8. fig. 3, where RM, RM are the 
muscles. At opposite ends of the long axis of the stomach the muscles are on opposite 
sides of the mesenterial plates. The mesenterial chamber (seen beneath in the drawing), 
which is free of muscles, is the “ Dorsalfach ” of Kolliker ; the opposite one the 
“ Ventralfach.” The muscles are covered by the entodermic layer, and are in direct con- 
tact with the median plates of the mesenteries, being modifications of the mesoderm. 
I have not been able to find any definite protractor muscles in Heliopora. I have, 
however, occasionally seen fibres on the surface of the mesenteries at the lateral 
margins of the atrium, coming apparently from the stomach-wall, which may prove 
to be such. In transverse sections I have seen no trace of such muscles. 
Ueliopora having commonly twelve so-called septa and eight mesenteries, a definite and 
regular relation of the eight septa to the twelve plications of the wall of the calicle 
might naturally be looked for ; none such, however, exists. As has been before stated, 
the number twelve is by no means constant ; and where twelve are present the 
arrangement varies in all kinds of ways. In Plate 8. fig. 3 the plications are shown in 
a section, and their relations are accurately copied. Here there may be counted 
either twelve or thirteen such plications, representing corresponding calcareous septa 
in the indentations. 
There are eight mesenterial filaments, as usual, present, which spring from the angle 
where the retractor muscles are inserted into the stomach-wall, and are continued 
down the free borders of the muscles, being attached to them. The filaments have 
the usual structure. Two filaments appear to be constantly longer than the others ; 
* Professor Scoteideb, and M. Eoitekeu must certainly Faye been mistaken in tbeir conclusions concerning 
tbe arrangement of the muscles with regard to the mesenteries in Alcyonaria, if the figure given in the ‘ Ann. 
& Mag. Nat. Hist.’ 1871, vii. p. 437, as representing them he correct. 
IsIDCCCLXXYI. Q 
