OCR Text |
Show 430 MR. E. W. L. HOLT-STUDIES IN [May 1, coming. We have seen, when examining fig. 3, that the orbital cavity is bounded by walls which are practically rigid. It would in fact be impossible for the eye to be retracted at all if the cavity had no such secondary chamber as is furnished by the recessus, and it is therefore by no means surprising that this structure is always most developed in the orbit in question. The position of the recessus of the upper eye on the side of the head to which this eye morphologically belongs seems to indicate that the organ is developed before the union of the ectethmoid and sphenotic of the blind side, a union which does not take place until after the eye has crossed the ridge1. Conclusions. The function of the recessus orbitcdis seems sufficiently clear, but its homologies must remain in doubt for the present. It is almost certainly homologous with the pouch-like diverticulum of the membranous wall of the orbit discovered by Dr. Giinther in Chorisochismus dentex. I have examined the examples of this fish which are contained in the National collection and have made a dissection of one. The organ exhibits no features not noted by Dr. Giinther, except that it is rather flattened in the case of m y specimen. It occupies a position immediately below the eye in a rather large subdermal cavity, which is plainly visible through the skin in all examples. The orifice by which it communicates with the orbital cavity is of moderate width, perhaps wider than in the case of any flat-fish which I have studied. Internally I could see no distinct muscular bands ; but the walls of the sac are rather stout, and appear to be muscular. As the specimen has been for many years in alcohol, which had no means of reaching the sac except through the tissues, it is quite possible that the internal parts may be to some extent altered by decomposition, and that muscular bands similar to those of the recessus in flat-fish m a y have originally been present. Slight pressure of the eye caused the discharge iuto the sac of a considerable amount of opaque yellowish matter, evidently decomposed tissue of some sort. It is evident, by comparison of the specimens, that the eye is capable of some vertical movement, and it appears most likely that the sac is functional in the same way as the recessus. The difference in position is merely such as might be brought about by the rotation of the eyes in Pleuronectids. This is plainly indicated in the posterior displacement which takes place in the choroidal notch of the lower eye in a metamorphosing flat-fish larva, and the recessus in the adult appears merely to have retained 1 Since this was written my attention lias been drawn by Professor Howes to some observations of Dr. Georg Pfeffer (Verb. Deutsch. zool. Gesell. 1894, p. 83), in which the formation of a bony orbital wall on the blind side of the upper eye is recorded as a regular feature in the development of the skull after that eye has completed its transit. |