1894.] TELEOSTEA^ MORPHOLOGY. 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 colour- 

 less 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. Tn 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 siugularly lung-like, the resemblance being even 

 more mai-ked 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 differ- 

 entiated. 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 natui'e 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 Y-cranial distinctly terminates on the outer face of the differ- 

 entiated area. 



The organ of the upper orbit, that belonging morphologically 

 to the blind side of the body, is saccular in all Pleuronectida which 

 I have examined, and is invariably larger than that belonging to 

 the ocular side. It is always situate on the blind side, communi- 



