The eye of _Branchiostoma_ is of the most rudimentary condition. It is simply a minute speck coated by dark pigment, and receiving the end of a short nerve. In _Myxinoids_ the minute rudiment of the eye is covered by the skin and muscles. This is also the case in many of the blind Teleosteous fishes; however, whilst in the former fishes the organ of sight has not attained to any degree of development, the rudimentary eye of blind Teleostei is a retrogressive formation, in which often a lens and other portions of the eye can be recognised. In fishes with a well-developed eye it is imbedded in a layer of gelatinous and adipose substance, which covers the cavity of the orbit. A lacrymal gland is absent. In the orbit of one fish only, _Chorismodentex_, an organ has been found which can be compared to a _saccus lacrymalis_. It is a round, blind, wide sac, of the size of a pea, situated below the anterior corner of the orbit, between the maxillary bone and the muscles of the cheek, communicating by a rather wide foramen with the orbital cavity. The membrane by which it is formed is continuous with that coating the orbita. In the Chondropterygians the eyeball is supported by and moves on a cartilaginous peduncle of the orbital wall. In the majority of Teleosteans, and in Acipenser, a fibrous ligament attaches the sclerotic to the wall of the orbit. The proper muscles of the eyeball exist in all fishes, and consist of the four _Musculi recti_ and the two _M. obliqui_. In many Teleostei the former rise from a subcranial canal, the origin of the _M. rectus externus_ being prolonged farthest backwards. The _Recti_ muscles are extraordinarily long in the Hammerheaded Sharks, in which they extend from the basis cranii along the lateral prolongations of the head to the eyes, which are situated at the extremities of the hammer. In all fishes the general integument of the head passes over the eye, and becomes transparent where it enters the orbit; sometimes it simply passes over the orbit, sometimes it forms a circular fold. The anterior and posterior portions may be especially broad and the seat of an adipose deposit (_adipose eyelids_), as in _Scomber_, _Caranx_, _Mugil_, etc. In many of these fishes the extent of these eyelids varies with the seasons; during the spawning season they are so much loaded with fat as nearly to hide the whole eye. Many Sharks possess a _nictitating_ membrane, developed from the lower part of the palpebral fold, and moved by a proper set of muscles. [Illustration: Fig. 48. Vertical section through eye of Xiphias. (After Owen.) _co_, Cornea; _sc_, sclerotica; _o_, nervus opticus; _c_, sclerotic capsule; _a_, membrana argentea; _v_, membrana vasculosa; _u_, membrana uvea; _ch_, choroid gland; _r_, retina; _f_, processus falciformis; _h_, humor vitreus; _l_, lens; _i_, iris.]