ZOOLOGY OF CHORDATES: STUDY GUIDE - Zakharenko M.O. - 2015

CHAPTER 4. SUPERCLASS TETRAPODA. LOWER TERRESTRIAL VERTEBRATES

The superclass unites vertebrates that have transitioned to terrestrial and aerial life. Some remain closely tied to Water throughout their entire lives or during specific periods, while only a few groups have secondarily returned to an aquatic existence, losing their direct connection to land yet retaining The ability to breathe atmospheric air (such as sea snakes and cetaceans). On land, these animals move using paired, jointed forelimbs and hindlimbs, the movement of which is powered by well-developed Muscles. Respiration intensifies, with gas exchange between the Organism and the environment occurring through Lungs. In amphibians, gills serve as the larval respiratory organ; in reptiles, birds, and mammals, they appear only during early embryonic development as gill slits, which rapidly regress. Two circulatory loops emerge—pulmonary and systemic—which become completely separated only in birds and mammals. The relative size of the Brain increases, accompanied by the differentiation of its distinct regions. Sense Organs are adapted to function in an aerial environment. Respiratory and olfactory Regions of the Nasal cavity develop, the Middle ear is formed, eyelids appear, and the shape of the eye lens changes; lateral line organs disappear, persisting only in larvae and certain adult amphibians.

The superclass Tetrapods comprises four classes: Amphibia (amphibians), Reptilia (reptiles), Aves (birds), and Mammalia (mammals). The latter three classes are grouped into Amniota—primary terrestrial vertebrates whose embryos, owing to special membranes, are capable of developing in an aerial environment. They are characterized by a High Metabolic Rate and a different type of Water-Salt METABOLISM. In terms of individual development, amphibians resemble fish and therefore belong to the group Anamnia—primary aquatic vertebrates whose eggs develop in water. An aquatic larva hatches from the egg and transforms into an adult animal through metamorphosis.

Birds and mammals are capable of maintaining a relatively constant and high body Temperature to a greater or lesser degree; hence, they are classified as warm-blooded (homeothermic or endothermic) animals, distinguishing them by this feature from poikilothermic or ectothermic vertebrates.