Comparative study of vertebrates

2. • It is a photoreceptor organ.
• Size and shape: Human eye is spherical about 2.5 cm in diameter.
• Location: It is situated of skull and is supplied by optic nerve.
• 3 coating of eyes: Scleroid , Choriod, Retina

3. The eye is first noticed in
week three of development.
The eyes are marked by the
appearance of the optic
placode and vesicle on either
side of the embryo. As the
anterior and posterior
neuropores close the optic
placodes it forms into the optic
vesicles.The optic vesicles
grow until they touch the
ectoderm of the skin of the
embryo. The lens then initiate
their formation, at which point
the vesicle evaginates to form
the optic cup.

4. By week six, the tissue that surrounds
the developing eye will transform into
an inner and outer layer. The inner
layer will become a tissue resembling
the pia mater, and the outer layer
resembles the dura mater. The pia
mater portion differentiates into the
choroid, whereas the dura mater
portion will become the sclera.
The anterior chamber forms when there
is degeneration of the tissue that yet
again forms an inner and outer layer.
The posterior chamber is bordered by
the iris and ciliary body. Both anterior
and posterior chambers are in open
communication with each other
through the pupil, and both contain
aqueous humor that continually
circulates.

5. Between the retina and the lens,
the vitreous body is formed which
surrounds the optic cup. The optic
cup has two layers: an inner and
an outer layer. The outer layer is
the pigmented layer of the retina,
and the inner layer is the neural
layer. In week seven, the optic cup
begins to form the pupil. The pars
optica retinae is the outermost
layer of the inner layer and will
contain the rods and cones. Next
to the pars optica retinae is the
mantle layer which develops into
the neurons of the eye.

6. • Hagfishes have rudimentary
eyes but Petromyzon has well
developed eyes.
• The eye ball is flat and eye
lids are absent.
• Lens is permanently
spherical and there are no
ciliary bodies.
• Sclera and choroid are fused
and pupil has fixed diameter.

7. • Eyelids are immovable and
permanently opened.
• In elasmobranchs, cornea is
flat in front, lens
permanently spherical
which is moved forward
and backward by a muscle.
There is a layer of shiny
guanine
crystals tapetum behind
retina for night vision.
Sclera is cartilaginous in
elasmobranchs.
• In bony fishes the eye ball
and lens is permanently
spherical that almost
touches the cornea in front.

8. Fig: Fish eye close up

9. • Limbless amphibians have
rudimentary eyes owing to
their burrowing habits.
• Urodeles also have small
eyes however, anurans have
evolved an eye that is
suitable for vision in
terrestrial conditions.
• Eyelids are poorly
represented in aquatic forms
but are well developed in
many terrestrial species.
• Lacrimal or tear glands are
present in many amphibians,
although they are poorly
developed.

10. Fig: Amphibian eyes

11. • Reptilian eye is made on amphibian plan
and is adapted for terrestrial environs.
• Most reptiles have a ring of bones around
each eye that supports the eyeball (the
sclerotic ring), crocodilians lack these
bones, just like mammals and snakes.
• Some lizards and tuatara contain a third eye
used for detecting changes in ambient light
conditions, called a parietal eye, located on
the top of their head. This eye contains a
cornea, lens, and retina like the lateral eyes,
but it is simpler in structure. Only two types
of neurons are in this eye: photoreceptors
and ganglion cells.
• Sclera is cartilaginous or bony and retina
possesses a foveacentralis.

12. • Lizards also have a conuspapillaris
protruding on the lower side of the fovea. It
probably has nourishing function.
• Lens is soft and flexible and can alter focal
length by squeezing with the help of cilliary
muscles.
• Lachrymal glands are well developed
except in snakes.
• Lower eye lid is still larger and nictitating
membrane is present.
• In snakes eyelids are fused and become
transparent to form a spectacle in front of
the eye.
• Crocodiles have a pecten as in birds. The
light receptors in crocodilians’ eyes include
both cones and numerous rods, so it is
assumed all crocodilians can see colors.
• Colour vision seems to be present in some
lizards and turtles.

13. Fig: Reptilian eye

14. • Bird eye is best developed among all vertebrates
which is a necessity for a flying animal.
• Bird eye ball is large and depressed in the area
of powerful cilliary body.
• Sclerotic coat is tough with cartilages and
bones. Retina is highly sensitive having
abundance of cones and a very
deep foveacentralis.
• A pecten is present in birds which increases
contrast and reduces glare in bright sunshine.
• Lens is highly flexible and can be squeezed by
two powerful muscles, namely, muscle of
Crampton and muscle of Bruke. Iris is
supplied with striated muscles.
• Nictitating membrane is used to cover the front
of eyeball when the bird is in flight. Except for
the nocturnal birds, all birds possess colour
vision.

15. Fig: Avian eyes

16. • The outermost layer of the eye ball is known as
tunica fibrosa, skeletal in function, and maintains
the rigidity of the eye ball.
• The sclera is opaque, and is supplied by optic nerve
and blood vessels, but in front, it is replaced by a
transparent layer called cornea.
• A thin mucous membrane, in continuation with the
eye lids, is present in front of the cornea. This is the
conjunctiva.
• Tunica vasculosa is composed of choroid, cilliary
body and iris, rich in blood vessels and pigment
cells. Iris can change the diameter of pupil
by radial and circular muscles.
• The innermost layer of the eye is retina. It consists
rods and cones which carries visual impulses.
Retina has a fovea but there is no pecten.

17. Fig: Human eye under microscope

18. • The retinal layer is higly innervated by the
optic nerve. The point where the optic nerve
enters the retina is known as the blind spot.
• Lens is soft and capable of changing focal
length by altering its thickness with the help
of circular and meridional muscles present
in the cilliary body.
• Upper eyelid is large in mammals and
lachrymal gland is located on the upper side
of the eyeball.
• Nictitating membrane is present in some
mammals such as horse, anteaters, panda and
caribou.
• A modification of iris called umbraculum is
found in cats, camels etc. which cuts
excessive glare in daytime.

19. Fig: A tiger and a bat.