VISION. XXIX 



extremities, as in the hammer-headed shark, Zygcena malleus. They may 

 be on elongated peduncles capable of retraction, as in some of the mud- 

 hunting gobies, Periophthalmus, Boleophthalmiis, and their allies, which in 

 tropical counti-ies pursue insects and other objects suitable for food over the 

 soft mud, into which they themselves even disappear when chased' by birds 

 or other enemies. While we have instances on our own shores in the 

 blennies, which appear to observe objects as well with their heads out 

 of the water as when submerged in the sea, and they move their eyes 

 independently one of the other. In the sunfishes (Orthag orisons) there exists 

 a circular palpebral fold provided with a sphincter, while some sharks have a 

 nictitating membrane. Among the flat-fishes (Pleuronectidce) are remarkable 

 modifications. In the very young as they emerge from the egg the fry 

 swims on edge similarly to other fishes, but after a longer or shorter period 

 the eyes are both found on the coloured side of the body. These two eyes, 

 even in the adult, can be moved independently of each other. That the 

 eyes in their earliest state are situated similarly to what obtains in the 

 remainder of the class of fishes is very interesting, as tending to show how 

 they have in course of time changed their position — a view still further 

 confirmed when we observe how it is not invariable for the two eyes to be on the 

 right and left side of all members of the same species. (See vol. ii, page 1-4.) . 

 Among the most perfectly constructed eyes in bony fishes the eyeball is 

 rarely quite spherical in shape, but presents the appearance of a flattened 

 bulb and a short axis. Among the semi-osseous forms, as the'elasmo- 

 branchs, it is supported by, as well as moves upon, a cartilaginous peduncle 

 passing from the walls of the orbit. 



The optic nerve may perforate the eyeball in its axis or obliquely to one 

 side ; for, should the eye be protrusible to ever so small an extent,, it neces- 

 sarily follows that the optic nerve must either be situated in a position where 

 protrusion of the eyeball would not stretch its delicate structure, or it must 

 be slightly bent or curved, in order to permit extension ; compressed as it 

 penetrates the sclerotic and choroid coats, subsequently it expands, ending 

 in the retina which lines about two-thirds of the inner chamber, leaying, 

 however, a margin, where its free borders meet, and through which passes a 

 fold of the choroid, sometimes having a dark pigmental layer. This portion 

 of the choroid, termed the falciform process, pushes before it a fold of the 

 hyaloid membrane or tunic of the vitreous humour. The falciform process 

 generally extends to the capsule, of the lens, to which it is firmly attached 

 by a substance termed campanula Halleri. 



The outermost covering of the eye, or sclerotic capsule, is of varying 

 thickness, fibrous or bony in Osseous, and cartilaginous in semi-osseous or 

 chondropterygian forms, as well as in sturgeons. Owing to the considerable 

 interspace existing between the sclerotic and choroid membranes, and which 



