Chordate Zoology - textbook - Y. V. Tsaryk - 2013

Chapter 5. SUPERCLASS TETRAPODS (LAND VERTEBRATES) TETRAPODA, seu QUADRIPEDA.

ANAMNIOTES (ANAMNIA) AND AMNIOTES (AMNIOTA)

5.2. CLASS REPTILES, OR REPTILIANS, REPTILIA

Reptiles are the first Class of true, primarily terrestrial vertebrates (Amniota). They lay large, yolk- and albumen-rich eggs covered by a tough, parchment-like shell. Fertilization is strictly internal. Embryonic development occurs in an aerial environment with the Histology/21.html">Formation of extraembryonic membranes—the amnion, serosa, and allantois; there is no larval stage. The hatchling differs from adults only in size. The dry Skin of reptiles is almost entirely devoid of glands. The outer layers of the epidermis undergo cornification, forming horny scales and scutes in the skin. Respiration is exclusively pulmonary. Well-developed Airways—the Trachea and Bronchi—are formed. Breathing is driven by the Movements of the rib cage. The Heart is three-chambered. Three vascular trunks arise independently from the ventricle, which is divided by an incomplete septum: two aortic arches and the pulmonary artery. The carotid Arteries, which supply arterial Blood to the HEAD, branch off only from the right aortic arch. The SYSTEMIC AND PULMONARY circulations are not completely separated, though their degree of Separation is higher than in amphibians. Excretion and Water balance are maintained by metanephric (pelvic) Kidneys. The relative size of the Brain is increased, particularly due to the enlargement of the cerebral hemispheres and the Cerebellum.

The Skeleton is bony, with some cartilaginous elements. The Axial Skeleton (spine) is divided into five regions. An elongated neck and the first two specialized cervical vertebrae (the atlas and axis) provide high mobility of the head. The Skull has a single occipital condyle, well-developed dermal bones, and temporal fossae, which are

Fig. 5.16. Ringed caecilian

framed by bony arches. The limbs are of the terrestrial type, featuring intercarpal and intertarsal joints. The Pectoral Girdle is connected to the axial skeleton via the Ribs, while the Pelvic Girdle articulates with the transverse processes of two sacral vertebrae.

Reptiles inhabit various terrestrial environments, predominantly in warm and often temperate latitudes; some species have secondarily returned to an aquatic lifestyle. Their overall metabolic and activity levels are noticeably higher than those of amphibians, although their body Temperature is variable and heavily dependent on the ambient temperature (poikilothermy).