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 higher 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 trabeculee 
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 
trabeculee 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 trabeculae. 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 coste. 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 trabeculae 
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 down 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 trabecule. Perforations placed at regular intervals 
