THE TELEOST EYE 581 



Descemet's mesothelium of the cornea. Histologically, it is usually com- 

 posed solidly of swollen or polyhedral epithelioid cells; but it may be 

 loculated and vascularized (Periophthalmus) or dotted with melano- 

 phores and iridocytes (Gadus). In the bluegill, Lepomis macrochirus, 

 the whole of the ligament appears to be occupied by a single lymph 

 sinus (perhaps continuous with the epichorioidal one), which is criss- 

 crossed by mesothelial trabeculae. Where the tissue is solid and epithel- 

 ioid, the cells contain granules which are perhaps always of glycogen, 

 perhaps sometimes of other substances. The ligament then has a 'secre- 

 tory' look; but what it may secrete, in the fashion of an endocrine gland, 

 is a puzzle. It could conceivably be the source of either the whole of the 

 aqueous humor, or of solutes which raise the osmotic pressure of the 

 aqueous; but the ligament is no less well developed in freshwater teleosts 

 than in marine ones. Occasionally, stuffed between the cornea and the 

 annular ligament, or sometimes embedded in the latter (but never in 

 the cornea or sclera), there is a 'canal of Schlemm', which has connec- 

 tions to iridic or hyaloid vessels and is obviously not homologous with 

 the true Schlemm's canal of the sauropsidans and mammals. 



The chorioid, in addition to the usual pigmented vascular layers, 

 choriocapillaris, argentea (usually) , and (occasionally) tapetum fibrosum, 

 characteristically contains the same 'chorioid gland' which we noted in 

 Amid — never as large, however, as there. It is ordinarily horseshoe- 

 shaped, though it sometimes forms a complete ring around the optic 

 nerve (some minnows), or may be divided in two parts as in one of the 

 sea basses (Labrax). Between the limbs of the horseshoe, ventral to the 

 optic nerve, there is a second body of the same histological sort — the 

 'lentiform body' — in some families and scattered genera. 



The presence of the chorioid gland is rigidly dependent upon the 

 presence of a 'pseudobranch', the vestigial hyoid gill which is found on 

 the inner side of the operculum or gill-cover in most teleosts. The blood 

 which has been aerated in the pseudobranch is gathered into an efferent 

 artery which, in the neighborhood of the optic nerve, enters the sclera 

 and breaks up into a set of capillaries in the chorioid gland. From these, 

 the blood flows into the ordinary chorioidal circulation. In fishes which 

 have lost the pseudobranch for any reason, the chorioid gland is in- 

 evitably lacking also. In general, this is true of small-eyed forms — 

 catfishes, eels, characins, elephant-fishes, etc. In a similar way, the 

 lentiform body is interpolated in the arterial supply to the falciform 



