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



of attachment to the under surface of the upper body- wall, and have their free border 

 more and more curved. The mesenteries of kieher order are attached to the outer 

 surface of the alimentary tube. The mesenteries are all perforated by several irregu- 

 larly oval apertures traversed in the recent coral by spines and calcareous trabecule 

 projecting from the faces of the septa. The mesenteries are provided with well-developed 

 muscular slips, which have rather a complicated arrangement within the major mesenteries. 

 Near the summits of these mesenteries the muscular fibres are directed nearly horizontally 

 outwards, stretching directly between the line of attachment of the mesentery to the 

 alimentary tube and the upper wall of the body. In the lower part of the mesentery the 

 fibres are disposed in curved lines crossing one another, but with a general downward 

 direction towards the base. Some of these vertical fibres are continued upwards so as to 

 cross the horizontal ones above them just described to some little extent, as shown in 

 the figure. The inferior border of the mesenteries overlies in the recent coral the costal 

 trabeculse as already described ; hence in a vertical section of the decalcified coral, such 

 as shown in the figure, a groove or hollow is seen beneath the lower border of the 

 mesenteries, left by the removal of the costal calcareous trabeculse. The lower borders 

 of the mesenteries are attached to a series of processes of soft tissue which join the basal 

 ridges of soft tissue lying between the costas. This series of processes is seen in the 

 figure at the base, and the processes are seen to be separated from one another by a 

 series of apertures, through which, in the recent coral, passed the calcareous trabeculse 

 seen in figure 5, Plate XVI. It is to this series of processes that the vertical muscular 

 fibres of the mesenteries are attached. They are gathered towards the lower borders of the 

 mesenteries into a series of distinct bundles which pass clown into these processes, and 

 hence the muscular arrangement towards the lower borders of the mesenteries appears very 

 complicated to the eye. Towards the inner regions of the lower parts of the mesenteries 

 some muscular fibres are directed almost horizontally inwards towards the columella. 



Most of the major mesenteries in the single specimen dissected bore large ova and 

 embryos at the upper parts of their free margins. Some of the embryos were in an 

 advanced stage of development, but were so far contracted by the action of reagents that 

 their form could not be satisfactorily made out ; nor could it be determined whether they 

 were free in the mesenterial chambers or still attached. In nearly all the mesenterial 

 cavities were found one or two small Crustacea (a Gammarid ?), which must apparently 

 live as commensals within the cavities of the living coral. 



Leptopenus, n. gen. 



Corallum discoid, excessively thin and fragile, with the wall so completely covered 

 by perforations as to resemble lace-work, being built up of a network of delicate radiat- 

 ing and circumferentially-directed trabecular Perforations placed at regular intervals 



