OCR Text |
Show 1894.] TELEOSTEAN M O R P H O L O G Y . 425 alcohol previous to dissection, owing to its great contractility in the perfectly fresh condition. In fresh examples the organ will be found to contain a colourless fluid, which becomes a milky-white coagulum when the fish is rather stale. The same fluid exists also in the orbital cavity, and it can readily be passed from one to the other by pressure. The organ, owing to its internal structure, will be found to be extremely elastic, and this elasticity is retained to some extent for a considerable time after death. In figure 5 the organ is shown in a moderate state of expansion, slight pressure being applied to the eye at the time of drawing. Inflated with air the organ becomes singularly lung-like, the resemblance being even more marked in the case of its fellow of the blind side. The organ is rather more developed in Pleuronectes than in Solea, otherwise there is no important difference between the two genera. In the Brill the organ of the lower eye is much smaller than in Pleuronectes and Solea. The membranous wall of the orbital cavity, otherwise undifferentiated, expands in a conical process behind the eye, and the apex of this process is furnished with internal muscular bands similar to those met with in the definite sac-like organ of Pleuronectes. There does not seem to be a definite narrow opening between the muscular apex and the rest of the conical process of the Brill, and the whole apparatus is not very conspicuous unless pressure is applied to the orbital cavity. The lower orbital cavity of the Halibut is destitute of a definite sac-like process, but a portion of its membranous wall is differentiated. On removing the skin behind the eye, more or less fibrous and a great deal of adipose matter is found to overlie the orbital cavity. Bemoving this, the membranous wall, otherwise translucent, is seen to exhibit a trihedral opaque whitish patch in the position occupied in Pleuronectes by the sac, or a little posterior thereto. It is seen that the orbital membrane is thickened by the development of a number of minute lobules or sacculi. Their saccular nature is easily proved by inflating the orbital cavity, which causes them to stand out distinctly. They collapse again as soon as the pressure is reduced, and cannot be expanded by merely depressing the orbit. Examination of the internal surface of this part of the membranous wall shows a complex arrangement of white muscular bands, forming a network pierced by numerous smaller and larger orifices, one, in a specimen examined, being considerably larger than the rest. The structure is thus essentially the same as that of the organ in Pleuronectes, the only difference being in the number of orifices by which the organ communicates with the general lumen of the orbital cavity. A small branch of the V-cranial distinctly terminates on the outer face of the differentiated area. The organ of the upper orbit, that belonging morphologically to the blind side of the body, is saccular in all Pleuronectids which I have examined, and is invariably larger than that belonging to the ocular side. It is always situate on the blind side, communi- |