Figure 13.20: A tuatara. ( 19 )Many of the viviparous species feed their fetuses through various forms of placenta, similar
to those of mammals, with some providing initial care for their hatchlings. The degree
of viviparity varies: some species simply retain the eggs until just before hatching, others
provide maternal nourishment to supplement the yolk, while still others lack any yolk and
provide all nutrients via a placenta.
All reproductive activity occurs with the cloaca, the single exit/entrance at the base of the
tail, where waste is also eliminated. Most reptiles lay amniotic eggs covered with leathery
or calcareous shells. An amnion (the innermost of the embryonic membranes), chorion (the
outermost of the membranes surrounding the embryo) and allantois (a vascular embryonic
membrane) are present during embryonic life. There are no larval stages of development.
Most reptiles reproduce sexually, although six families of lizards and one snake are ca-
pable of asexual reproduction. In some species of squamates, a population of females is
able to produce a nonsexual diploid clone of the mother. This asexual reproduction called
parthenogenesisoccurs in several species of gecko, and is particularly widespread in the
teiids and lacertids.
Extant reptiles range in size from the newly-discovered Jaragua Sphaero, at 1.6 cm (0.6 in),
to the saltwater crocodile, at up to 7 m (23 ft).
Most reptiles have a closed circulatory system with a three-chambered heart consisting of
two atria and one ventricle. All reptiles breathe using lungs, although aquatic turtles have
developed more permeable skin, and some species have modified their cloacas to increase the
area for gas exchange. Excretion is performed mainly by two small kidneys.