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Animal Diversity: Chordata

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Animal Diversity: Chordata Animal Diversity: Chordata Anamniotes Amphibia Dr. Monisha Khanna Acharya Narendra Dev College University of Delhi Kalkaji New Delhi – 110 019 [email protected] List Of Contents I. Introduction II. Classification A. Order 1. Apoda (Gymnophiona / Caecilia) B. Order 2. Urodela (Caudata) C. Order 3. Anura III. Origin A. From Early Chordates To The First Land Vertebrates a. Origin of Chordates b. Origin of Vertebrates c. Origin of Tetrapods B. Acquisition of Adaptations For Life On Land IV. Amphibia: General Organization a. External Appearance b. Integument c. Alimentary Canal d. Respiratory Organs And Voice Apparatus e. Blood Vascular System f. Endoskeleton g. Nervous System And Sensory Organs h. Urinogenital System And Osmoregulation V. Parental Care In Amphibia A. Order Anura B. Order Urodela C. Order Apoda I. INTRODUCTION Members of the phylum Chordata are commonly referred to as chordates. Four characters, of prime diagnostic importance, are possessed by all chordates: 1) A primitive endoskeletal structure called the notochord is present during early embryonic life. This pliant, rod-like structure, composed of a peculiar type of connective tissue, is located along the mid-dorsal line, where it forms the axis of support for the body. In some animals it persists as such throughout life, but in most chordates it serves as a foundation around which the vertebral column is built. 2) A hollow, dorsal nerve tube is present sometime during life. The central nervous system, made up of the brain and the spinal cord, is located in a dorsal position just above the notochord. It is a hollow canal from one end to the other. 3) Gill slits or traces of them connecting to the pharynx are present at some stage of life. Most aquatic chordates respire by gills made up of vascular lamellae or filaments lining the borders of the gill slits, which connect to the pharynx and open directly or indirectly to the outside. Even terrestrial chordates, which never breathe by gills, nevertheless have traces of gill slits present as transient structures, during early embryonic life. No vascular lamellae line these temporary structures, nor do they open to the outside, but the fact that they are present in all chordates is of prime importance in denoting close relationship. 4) Chordates possess a post-anal tail in some stage of their life that represents a posterior elongation of the body extending beyond the anus. The tail is primarily an extension of the chordate locomotor apparatus, the segmental musculature and notochord. Apart from the above four features, chordates also have certain characteristics common to some other phyla as well. 5) They are bilaterally symmetrical; 6) are metameric; 7) have a true body cavity or coelom, lined with mesoderm; 8) show cephalization or the concentration of nervous tissue and specialized sense organs in or towards the head; 9) the blood is pumped anteriorly from the ventrally located heart and forced to the dorsal side. It then moves posteriorly and returns to the heart by veins. The phylum Chordata is usually subdivided into four main groups or subphyla. The first three of these include a few relatively simple animals, which lack a cranium and brain. These organisms are sometimes collectively referred to as the Acrania. The animals included in this category are believed to show similarities to the chordate ancestors, hence are frequently known as the protochordates. These are the subphyla Hemichordata (acorn worms), Urochordata (tunicates), and the Cephalochordata (amphioxus). Vertebrata (Craniata) is a large group, embracing chordates having a brain; endoskeleton; notochord not extending forward under the brain; paired eyes; presence of red blood cells; a ventrally placed heart; presence of a sympathetic nervous system; and presence of a hepatic portal system. Vertebrates include the jawless forms, lacking vertebrae (Super class Agnatha), and the jawed vertebrates (Super class Gnathostomata). Furthermore, the latter include the series, Pisces embracing the lower forms commonly known as fishes. The remaining vertebrates are included in the group Tetrapoda, which are basically four-footed animals, although in some the limbs have been lost or modified secondarily. Tetrapods are those members of the subphylum Vertebrata having paired appendages in the form of limbs rather than fins, though in some forms the limbs either degenerate completely or show modifications. Among other characteristics which distinguish tetrapods from fishes are a cornified outer layer of skin; nasal passages which communicate with the mouth cavity and which transport air; lungs used in respiration; and a bony skeleton along with a reduction in the number of skull bones. 2 Tetrapoda The class Amphibia is composed of tetrapods in which the transition from aquatic to terrestrial life is clearly indicated. Amphibians are the first vertebrates to live on land, although they lay their eggs in water or in moist situations. The first tetrapods evolved from rhipidistian crossopterygian fishes. The fossil remains of primitive tetrapods have been found in the eastern parts of Greenland in Devonian deposits. These specimens have features intermediate between late crossopterygians and early amphibians. The group Tetrapoda is divided into four classes made up of amphibians, reptiles, birds and mammals. The living representatives of the class Amphibia include salamanders, newts, frogs, toads, and the caecilians. The amphibians lead a double life, that is, first in the water, and then on the land. The result of this ambitious attempt is that they present a medley of makeshift adaptations, which leave them still a long way from vertebrate perfection. Among the dual adjustments that they make, are those associated with locomotion and protection against desiccation. In water, an elongated fishlike body, propelled by a muscular tail, has proved to be the most efficient mechanism for locomotion. However on land, the weight of the body is no longer supported by the surrounding aqueous medium, so that the two pairs of appendages become modified into legs, which act as levers to lift the body away from the ground. Such levers are equipped with adequate muscles without adding excessively to the body weight. However the amphibians are not particularly successful at locomotion on land. Even in frogs and toads, where amphibian legs reach their highest development, such locomotor appendages are so inefficiently anchored to a single vertebra of the supporting backbone that these animals cannot bear their weight upon them in the sustained manner necessary for standing or walking, and can progress only by the momentary exertion of hopping or jumping. The problem of dessication arises from the fact that the surrounding air, takes up moisture rapidly from any moist surface. Amphibians not only utilize gills and primitive lungs in respiration, but also exchange gases to a very large extent directly through the skin. Consequently, these animals can live only in moist places. In comparison, the higher land animals, in which an efficient pulmonary system is formed, are not restricted because they develop a thick, relatively dry integument, which is resistant to dessication. Thus, relatively 3 inefficient respiratory organs, together with other anatomical handicaps prevent amphibians from maintaining a body temperature independent of that of the surroundings. The difficulty of avoiding dessication is also involved in the breeding habits of amphibians because they have not made the changes required of true land vertebrates. No amnion (liquid- filled sac) is produced by the embryos of lower vertebrates including the Amphibia. The latter must therefore go back to the water to breed in most cases. Furthermore, the metamorphosis of such an amphibian as a frog or a toad, necessitated by its emergence from water to land, works profound changes both in its structure and in its feeding habits. For instance, during its lifetime a toad changes its diet six times. While in the egg it absorbs the yolk; upon hatching it develops a temporary mouth and eats the jelly of the egg envelopes; next it becomes the free swimming tadpole feeding mainly upon the aquatic vegetation; the juvenile stage has fat bodies provided to meet the intervening demands of hibernation; with the warmth of spring the young toad catches slugs and insects for a living. The distinct features of amphibians can be summarized as follows: 1. Amphibians are ectothermal vertebrates. 2. They have varied body forms – ranging from elongated forms, with a distinct head, trunk and tail; to a compact, depressed body with a fused head and trunk and no intervening neck. 3. Limbs are usually four in number, although some forms are limbless. 4. Skin is smooth and moist with many glands including pigment cells. Poison glands are sometimes present but scales are mostly absent. 5. Mouth is usually large, with small teeth in either upper or both jaws. Teeth are bicuspid and pedicellate. In some forms, teeth are completely absent. The nostrils open into the anterior part of the mouth cavity. 6. Skeleton is mostly bony, with varying number of vertebrae; ribs are present in some forms but absent in others. Ribs if present do not encircle the body. Centra of vertebrae are cylindrical. Similar type of vertebra is also found among several groups of early tetrapods. There is the presence of double or paired occipital condyle. The posterior skull bones have been lost. Small, widely separated pterygoids are found. A small bone in the skull called operculum is present and is fused to the ear bones in most anurans; it is perhaps involved in hearing and balancing. 7. Ability to elevate the eye with specially developed levitator bulbi muscle. There is also the presence of a special type of visual cell in the retina known as the green rod. (This however is absent in Apoda). 8. Respiration occurs by lungs, skin and gills, either separately or in combination. A forced pump respiratory mechanism exists. The larval forms have the external gills that may persist throughout life in some forms.
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