The vascular tunic of the eyeball of both Oreochromis niloticus and Mugil cephalus consists of a posterior choroid and an anterior iris. The choroid consists mainly of a vascular layer bordered externally by the choroidal argenta. The latter is represented by a light brown, more or less uniformly thick layer of fusiform or rod shaped cells separating the choroid from the sclera. Unlike that of Mugil cephalus, the argenta of
Assiut Vet. Med. J. Vol. 52 No. 110 July 2006
Oreochromis niloticus is pigmented. The vascular layer of the choroid appears in the form of one layer of thin walled vessels that widen posteriorly forming a vascular plexus; the choroid gland or Rete mirabile, which is suggested to have a haemodynamic function. The iris is the anterior continuation of the choroid with which it is similar in general structure. The base of the iris is covered anteriorly by a layer of flat epithelium and posteriorly by a bilayered epithelium. The inner one of the latter is non-pigmented, but the outer layer is pigmented. Near the pupillary margin, the iris of Mugil cephalus shows a thin layer of smooth muscle cells that may be an accommodation for the brackish water habitat of this species of fishes.