20 THE SNAKES OF SOUTH AFRICA. 
hour or so daily into the cages, owing to their situation. In 
consequence the female puff adders frequently produce their 
young in an incomplete state of development. In others the 
embryo dies and the egg hardens, resulting in the death of the 
parent. 
Practically speaking, then, snakes are oviparous and_ vivi- 
- parous, that is, some lay eggs and others give birth to young. 
SENSE OF SMELL AND BREATHING APPARATUS. 
The sense of smell in snakes is very well developed, as will 
be seen on dissection of the apparatus of thenose. In the family 
of grass snakes this sense of smell is particularly good. 
Snakes are thus enabled to find their prey and guard against 
their enemies by the senses of sight, touch, smell, and hearing. 
Snakes breathe by means of lungs. The left lung is much 
smaller than the right one, and in most cases it is quite rudi- 
mentary, or entirely absent. In some snakes the right lung 
reaches from the neck fully two-thirds of the way down the body 
of the snake. The lung is a sort of long hollow tube or bag, with 
thin walls, in which are embedded the cells and blood vessels 
which take up the oxygen of the inspired air. _ Snakes, owing to 
their low temperature and slow blood circulation, do not require 
to breathe as frequently, or inhale such pure air as birds or mam- 
mals. The breathing is slow and quiet, except when the serpent 
is alarmed or enraged, when it will inhale a great volume of air 
and expel it forcibly, producing the characteristic hiss. 
HISSING. 
The hissing of a snake is caused by the long sac-like lung 
being inflated with air, which is forcibly expelled through the 
glottis and nostrils, causing that well-known hissing sound which 
warns us of the proximity of a snake. 
The Puff Adder makes the loudest and most prolonged hiss of 
any South African snake. Hissing is the only sound snakes are 
capable of producing, except the American Rattle Snakes, which 
have a horny substance in loose sections or segments at the end 
of the tail, which, when shaken, emits a hollow kind of rattling 
noise. 
