116 THE VOYAGE OF H.M.S. CHALLENGER. 



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 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 (PI. 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. II. 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 



