116 THE VOYAGE OF H.M.8. CHALLENGER. 
opposite sides of the mesenterial plates. The mesenterial chamber (seen beneath in 
the drawing), which is free of muscles, is the ‘ Dorsalfach” of Kélliker; the opposite 
one the ‘ Ventralfach.” The muscles are covered by the endodermic layer, and are in 
direct contact 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 of 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. 
Heliopora having commonly twelve so-called “septa” and eight mesenteries, a definite 
and regular relation of the eight mesenteries 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 arrange- 
ment varies in all kinds of ways. In Plate I. fig. 3 the plications are shown as seen in 
an actual section, and their relations are accurately copied. Here there may be counted 
either twelve or thirteen such plications, representing corresponding calcareous septa 
which occupied 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 ; 
but I am uncertain about this point, it being very difficult to get a view of all the 
filaments uninjured in any one polyp. To which sides of the mesenterial plates the 
filaments are attached I have not made out. 
Generative Organs.—Out of at least a hundred polyps examined from the colony of 
Heliopora hardened for examination, only three were found to contain generative 
organs ; in each of the cases ova. In two of the polyps a single ovum only was 
present, in the third four ova attached singly to four mesenteries. The ova are attached 
to the edges of the muscular margins of the mesenteries at a point about halfway 
between the origin and insertion of the fibres composing the lower border of the 
muscle (Pl. I. fig. 1). The ovum is attached to this border by a specially developed 
mass of endodermal cells, and at its point of attachment is in close relation with the 
mesenterial filament. The ova, as shown (PI. IL fig. 8), are large, measuring 
‘21 mm. in diameter (the smallest observed measured ‘17 mm. in diameter); they 
are composed of an outer membranous capsule, by means of which they are attached 
in position, and a contained mass of yelk-globules, in which lies a germinal vesicle and 
germinal spot. 
It was not determined which of the mesenteries bore ova, or whether those with long 
filaments bore them or not, the expectation that abundance of fertile polyps would be 
