204 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
There are no glands connected with the eyes in cyclostomes or 
fishes, but in the urodeles a series of glands is developed from the con- 
junctival lining of the lower lid. In the amphibia they show little 
differentiation, but in all sauropsida (glands are lacking from a few 
reptiles—crocodile tears are non-existent) they become divided into 
two groups. One becomes aggregated near the inner angle and forms 
what is known as Harder’s glands (glandula membrana nictitans); 
the others migrate toward the outer angle of the eye and constitute the 
true lacrimal or tear gland. In the mammals the migration continues 
until the gland comes to lie beneath the upper lid, where it shows its 
multiple nature by the numbers of ducts by which it pours its secretions 
into the conjunctival sac. In most mammals Harder’s gland degener- 
ates. The tears secreted by the glands pass over the conjunctiva and are 
collected at the inner angle of the eye, where they are drained by the 
lacrimal duct into the cavity of the nose. This duct is formed as a 
thickening of the epidermis which later becomes perforated. It 
follows the course of an earlier groove (fig. 194) leading from the orbit 
to the nasal invagination and which was formerly thought to form the 
duct. 
The eyes of the cyclostomes are degenerate. In the larval (Ammoccetes) stage 
of Petromyzon the eye is buried under a thick skin, but this thins out in the adult. 
In the myxinoids the lens and eye muscles are lacking, and iris, cornea and sclera 
are not differentiated. ; 
Fishes have eyes with a very flattened cornea, a spherical lens and very long 
retinal rods. A peculiar feature in many fishes is the falciform process, a vascular 
and muscular structure which enters the retinal cup through the chorioid fissure and 
extends to the lens where it bears an expansion, the campanula Halleri. The 
whole is supposed to act as a means of accommodation, there being no ciliary 
muscles. In most fishes the eyes are so placed on the sides of the head that there 
must be monocular vision. In the flat fishes (Heterosomata) one of the eyes mi- 
grates during development, so that both eyes come to lie on one side of the head. 
Most sauropsida are characterized by the development of a process from the 
inner retinal surface which reaches its extreme in the pecten of the birds. In the 
reptiles it is a small conical process arising from the point of entrance of the optic 
nerve, but in the birds this expands distally into a quadrangular plate, folded like a 
fan, to which various functions have been ascribed. It has been recently shown 
to be rich in sense cells. The shape of the eye of the bird is peculiar, but is not 
easily described. It consists of a hemispherical posterior part, followed by a 
conical portion, and this surmounted by a hemispherical corneal region, the whole 
being somewhat telescopic in shape. The whole is very large in proportion to the 
size of the animal. 
The pecten is said to be outlined in the foetal stages of some mammals. The 
