Animal Diversity: Chordata

not folded to any extent. In some forms, a shelf-like fold from the lateral wall foreshadows
the appearance of the conchae or the turbinal folds, which become highly developed in more
advanced vertebrates. Glandular areas in the nasal passages keep the olfactory epithelium
moist. In addition, a new olfactory structure first appears in amphibians and is known as the
Jacobson’s / vomeronasal organ, which communicates with the olfactory chamber and the
buccal cavity. It arises as a ventro-medial or ventro-lateral evagination of the nasal passage
and is believed to be used in testing food substances held in the mouth. It is supplied with
branches of the terminal, olfactory and trigeminal cranial nerves.
Gustatoreceptors: Taste buds of amphibians are found on the roof of the mouth, tongue and
on the lining of the jaws. In frogs, they lie on the free surfaces of the fungiform papillae. The
more numerous filiform papillae do not have taste buds associated with them.
Lateral line sense organs occur in the larval stage and these are receptive to water vibrations.
Branches of facial, glossopharyngeal and vagus nerves supply these. These sensory structures
however disappear during metamorphosis.
The Visual sensory organs: Among amphibians, the anuran eyes are best developed and are
lodged in the orbital fossae on either side of the head. The upper eyelid is large and
immobile, while the lower eyelid forms a protective nictitating membrane, which covers the
eye whenever the animal is in water (Figure 19). The movement of the nictitating membrane
is regulated by the retractor bulbi and the levator bulbi muscles. The eyeball consists of three
concentric layers: the outermost fibrous tunic, middle vascular uvea and the innermost
nervous retina. The large posterior part of the fibrous layer is called the sclerotic while the
small anterior transparent part in front is the cornea. The curved surface of the cornea helps
the lens in focusing the light rays. Uvea is made of loose connective tissue having blood
capillaries and pigment cells. Its part lining the sclerotic is called the choroid. At the junction
of the sclerotic and cornea is the ciliary body but ciliary muscles are absent. The uvea in front
separates from the sclerotic to form the pupil, which is perforated in the middle by an
aperture called the pupil and has sphincter and dilator muscles to regulate the amount of light
entering in the eye by changing the size of the pupil. The iris divides the cavity of the eyeball
into a small anterior aqueous chamber and a large posterior vitreous chamber.
The innermost delicate part of the eye is called the retina. Its optic part consists of an outer
pigmented layer adjacent to the choroid and the inner nervous layer having receptor cells and
neurons. The inner sensory area has three regions: the outer light-sensitive cells, the middle
bipolar nerve cells and the inner ganglion cells. Rods and cones are found in the light
sensitive cells and contain the visual pigments, rhodopsin and iodopsin. An area called the
macula lutea or yellow spot is the small part of retina just opposite the center of pupil and is
the point of most acute vision. However, frog eye lacks the fovea centralis depression. The
lens is enclosed in a delicate lens capsule and lies just behind the iris.
The light rays entering the eyeball are focused on the retina by a combination of the
conjunctiva, cornea, aqueous humor, lens and vitreous humor. The inverted image formed on
the retina stimulates the receptor cells in the area centralis, which in turn generates nerve
impulses that are conveyed to the brain by the optic nerve. The eyes of a frog have limited
focusing power. The frog is short sighted on land but far sighted in water.
Auditory sense organs: The ears of frog located behind and below the eyes are organs of
hearing and equilibrium. Each ear consists of the middle and the inner ear. External ear is
absent. The middle ear is visible from outside and encloses an air filled cavity called the
tympanic membrane. It is limited internally by the auditory capsule and externally by the
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