have an epidermally derived, loosely segmented,
hollow rattle that vibrates when the tail tip is
moved rapidly (16.35). A new button-like segment
is added each time the rattlesnake moults its skin.
Embedded osteoderms are present in the skin of
many reptiles: for example, on the dorsal and lat-
eral surfaces of crocodilians and on the limbs of
some tortoises. The bony box-like shells of most
chelonians are covered with keratinous plates
(16.36 and 16.37). Even the soft and leathery
shells of soft-shelled turtles are covered with
lightly keratinized squamous epithelium. Snakes
shed their senescent epidermis periodically as
inverted tubes of (usually) unbroken old skin from
which they crawl. Lizards, crocodilians, the
tuatara and chelonians moult their old epidermis
272
piecemeal. Aquatic turtles shed one or more of
their hard outermost layers of shell plates period-
ically. Many tortoises merely add more keratin
concentrically to their plates (or scutes) as they
grow throughout life.
Reptiles lack sweat glands. However, some lizards
have a few sebaceous glands. In some species, modi-
fied sebaceous glands secrete holocrine, pheromone-
rich waxy substances that are important inducers or
releasers of sexual or territorial behaviour (16.38<
16.40). Other scent-producing secretions are elab-
orated by cloacal and hemipenial sheath glandular
structures (see Chapter 11, 11.41) and by the sex-
ual-segment granularity observed in the distal con-
voluted tubules of some male lizards and snakes (see
Chapter 9, 9.21).
16.35 Rattle of a small Mexican
rattlesnake (Crotalus enyo). This
keratinized structure forms as a
button-like protruberance to which a
loosely interlocking segment is added
each time the snake moults its
integument. The most cranial
segment retains a living core of
dermis, whereas the tissues of the
distal segments are no longer living.
H & E. ×12.5.
16.35
16.36 The carapacial shell of a red-
eared slider turtle (Trachemys scripta
elegans), is covered by a layer of
horn-like keratin (1) and a variable
thickness of stratified squamous
epithelium (2). The dermis is variable
in thickness, depending upon the size
and species of the turtle. It covers
multiple layers of membranous
compact and cancellous bone in
which bone marrow fills the
cancellous spaces. H & E. ×62.5.
16.36
1
2
Comparative Veterinary Histology with Clinical Correlates