1889 - 90 .] Mr W. L. Calderwood on Dactylopterus volitans. 133 
swimming bladder in Trigla is well known ; there, on opening the 
abdominal wall, it is distinctly seen projecting downwards from the 
dorsal wall of the abdominal cavity. In Dactylopterus, when the 
viscera are removed, only the ventral surface of the bladder is seen 
forming a part of the dorsal boundary of the abdominal cavity. 
This portion is white and tendinous, having on each side the com- 
mencement of a powerful muscle (fig. 1). The kidneys overlap it 
posteriorly, and the oesophagus and pericardial cavity partially 
obscure its anterior end. On removing the dorsal muscles of the 
trunk so as to view the bladder from the side and back, the bladder 
muscles are seen to continue upwards, to curve inwards towards the 
median line, to be reflected downwards on each side of the neural 
plate, and finally to become attached to the bodies of the verte- 
brae, whose spines go to form that plate. In this way the bladder 
becomes divided longitudinally into its two lateral portions. A 
careful examination, however, reveals more than this. On tracing 
the dorsal surface forwards by removing the superficial backward 
prolongation of the skull-sheath, a secondary division of the bladder 
is disclosed. It is composed of an extremely thin transparent 
membrane, is triangular in shape, its dorsal surface being so super- 
ficial as only to be covered by the dorsal plate of the shoulder. 
The base of the triangle is towards the median line, the other two 
sides are surrounded by bone, the ventral surface also lies in a bony 
cup ; so that this whole portion, although composed of an extremely 
thin membrane, is completely protected by rigid surroundings. 
Moreover, one-half of it may be fairly said to be within the region of 
the skull, since a section taken through the plane of the foramen 
magnum would, as nearly as possible, bisect the triangle. As seen 
in fig. 2, the posterior margin is the only one not surrounded by 
bone. Here it is that the primary division of the bladder communi- 
cates with this smaller secondary portion, that the muscular coat of 
the one thins out to give place to the thin membrane of the other.. 
Making now a vertical section through the long axes of the primary 
portion on one side, so as to obtain a view of the interior, we see 
that there is a thin membrane dividing it transversely into two — 
that in this membrane there is a foramen situated towards the inner 
margin. Immediately behind this membrane there is a tunnel seen 
passing in a transverse manner below the vertebral column, forming 
