424 MB. B. w. L. HOLT — STUDIES IN [May 1, 



The organ of the lower orbit — i. e., that belonging to the ocular 

 side of the body, in this case the right side — may be found 

 immediately under the skin overlying the membranous wall of the 

 orbital cavity just behind the eye. To expose it without rapture 

 requires rather careful handling, as, in the fresh condition, the 

 organ is extremely delicate, and it is bound to the skin anteriorly 

 by connective tissue. It is generally of a roughly trihedral out- 

 line, occupying a space bounded dorsally by the backward pro- 

 longation of the interorbital ridge of the skull, anteriorly by the 

 eye, and postero-ventrally by the anterior face of the superficial jaw- 

 muscles of the same side. To expose it fully it is necessary to 

 remove a part of these muscles. The organ wUl then present 

 much the appearance shown in figure 5 (Plate XXX.), according to 

 the state of expansion. Various branches of the V-cranial nerve, 

 which, for the sake of clearness, are omitted from the figure, will 

 be seen in its neighbourhood, mostly passing internal to it. The 

 distribution of these branches varies somewhat in the diiferent 

 species, but usually two small branches pass to the external face of 

 the organ, which receives its nervous supply from one of them. In 

 the Sole the course of the nerves not infrequently affects the 

 shape of the sac, which may assume a bilobate or even a trilobate 

 appearance therefrom. 



The sac is almost translucent in very fresh examples, save for 

 certain milky-white streaks observable about the periphery. It is 

 smooth externallj', and firmly bound on its inner face to the wall of 

 the orbital cavity. If the internal structure is exposed, by a partial 

 removal of the outer wall, as shown in Plate XXX. figure 6, it will 

 be seen that a number of white muscular bands are attached to the 

 inner surface of the walls, which are otherwise extremely thin and 

 delicate, while other bauds traverse the lumen, either independently, 

 or so associated together as to form more or less definite septa. 

 The bands on the walls, while they interlace with each other in 

 all directions, show nevertheless some attempt at a radiate dis- 

 position around an orifice which communicates with the orbital 

 cavity itself. There may be more than one such orifice ; in fact, 

 in the specimen figured there are two of considerable size (o., o.), 

 while several smaller ones are hidden by the septum (s.). Even 

 in cases where only one distinct orifice occurs, which is perhaps the 

 commonest condition in the Plaice and Common Sole, I am not 

 certain that minute openings do not also occur in its neighbour- 

 hood. The larger opening, or openings, always show a distinctly 

 thickened rim. The specimen figured appeared to be completely 

 subdivided by the septum, but this is not of constant occurrence 

 in the species. Indeed, in speaking of the dextral forms which 

 I have studied, it is only possible to say of the internal structure 

 of the accessory organ of their lower orbit that it is more or less 

 subdivided by muscular septa, and communicates with the orbital 

 cavity by one or more openings, placed close together, of which 

 one is always distinctly larger than the rest. It is advisable, in 

 studying this structure, to partially harden it by an injection of 



