1894.] TELEOSTBAN MORPHOLOGY. 429 



more potent than the relaxation of the recti and oblique muscles 

 to re-effect its protraction, and here again the elastic nature of the 

 recessus completely supplies the want. 



The actiou therefore, except in so far as it is, or may be, con- 

 cerned in the deposition of the orbital fluid, is purely mechanical, 

 and appears to be almost, if not quite, involuntary. I qualify the 

 statement thus because it appears to me that flat-fish have their 

 eyes very slightly more elevated when expectant of food than at 

 other times. A fish of the genus Pleuronectes usually shows that 

 it anticipates food by bending its body into a crescent shape, the 

 head and tail off the ground, ready for an instant dart; but the 

 greater degree of elevation of the eyes under such circumstances 

 may in reality only signify that a slight contraction of the orbital 

 muscles is characteristic of the resting condition. 



An involuntary organ might be expected to show some connection 

 with the sympathetic system, but such a connection I have not 

 found, though I am far from stating that it does not occur. The only 

 nervous supply with which I am acquainted is that derived from 

 the V-cranial, but whether from sensory or motor roots, or 

 from both, is a point which I have not yet investigated. We 

 have seen that the organ is most developed in Pleuronectes, 

 moderately so in Solea, and least developed in Rhombus ; and I can 

 say from observation that the power of elevating the eyes is pre- 

 cisely in the same proportion in those three genera. I have had 

 no opportunity of studying the habits of ffippoghssoides, as the 

 Long Rough Dab is very difficult to obtain in a healthy condition. 

 The habits of the Halibut are also unknown to me in this respect ; 

 but the fact that it shows the maximum development of the left 

 or upper accessory organ and the minimum development of the 

 right or lower organ, amongst the series of forms which I have 

 studied, is very probably due to the greater difference in the 

 level of its eyes, or of its oi'bital cavities, than in other flat-fishes. 

 The ocular surface of the head is very convex, while the left eye 

 never gets far beyond the ridge, and is consequently at a much 

 lower plane than the right eye. To bring the two eyes to the 

 same level, if that condition is actually attained by the living fish, 

 must certainly need a greater inequality in the elevating apparatus 

 than prevails in any other flat-fish known to me. 



The fact that the recessus of the eye belonging morphologically 

 to the blind side is the larger of the two in all the species studied 

 is susceptible of a very simple explanation. The eye of the ocular 

 side, the lower eye, has its orbital cavity bounded on the outer 

 side in great part by loose skin and connective tissue, thus allowing 

 whatever elasticity may be possessed by the undifferentiated mem- 

 branous wall in this region to come into full play when the eye- 

 muscles are contracted. The elasticity of the skin, combining 

 with the pressure of the external element over a surface greater 

 than that of the eye itself, must certainly afford some assistance in 

 elevating the eye as soon as the muscles are relaxed. 



In the case of the upper orbit, however, no such aid is forth- 



