198 ANIMAL FORMS 
ebb. Almost all the movements and operations of the body 
appear to be carried on by the animal with little apparent 
thought. Their acts, like most of the animals below them, 
are said to be instinctive; yet they are sufficiently well done 
to enable the animal to procure its food, avoid its enemies, 
and lead a successful life. As is true of other animals, the 
ability of the reptile to cope with its surroundings depends 
to a great extent upon the keenness of one or all of its or- 
gans of special sense. In the reptiles the sense of sight is 
perhaps sharpest, but there is considerable variation in this 
respect. Eyelids are present in all except the snakes, to- 
gether with a third, known as the nictitating membrane, a 
thin, transparent fold located at the inner angle of the eye, 
over which it is drawn with great rapidity. In the snakes 
eyelids are absent, giving the eye its characteristic stare. 
Furthermore, their sense of sight, except in a few tree-dwell- 
ing species, appears to be defective, the majority depending 
largely upon the sense of touch. 
In all the vertebrates a very peculiar organ known 
as the pineal gland or eye is situated on the roof of the 
brain. In several lizards its position is indicated by a trans- 
parent area in one of the plates of the head, and by an 
opening in the bones of the roof of the skull. In young 
reptiles, and especially in one of the New Zealand lizards 
(Hatteria, Fig. 119), its resemblance to an eye is decidedly 
striking. Lens, retina, pigment, cornea, are all present 
much as they are in some of the snails, but they finally 
degenerate more or less as the animal reaches maturity. 
It is a general belief that it represents the remnant of an 
organ of sight, a third eye, which looked out through the 
roof of the skull in some of the ancient vertebrates. 
With the possible exception of the few species of reptiles 
which produce sounds, probably to attract their mate, the 
sense of hearing is not particularly well developed. The 
senses of smell and taste are also comparatively feeble. The 
latter sense is located in the tongue, which is also popularly 
