430 MR. B. 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 ridge l . 



Conclusions. 



The function of the recessus orbitalis 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 my 

 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 may 

 have originally been present. Slight pressure of the eye caused 

 the discharge into 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 has 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. 



