Comparative anatomy of chordates - /2020-2021 Lecture 1 Introduction The students of chordate anatomy should realize at the beginning that all the methods of approach are based in part upon comparative anatomical relationships. Comparative anatomy may also be used to explain some of the phenomena that appear during embryonic development. A knowledge of structure alone, however, has very little meaning unless it is interpreted in terms of function. Morphology and physiology complement each other. So far as is possible, it is desirable that both structure and function be considered together in studying animals. Nevertheless, a preliminary knowledge of morphology is a great aid to the study of physiology. Comparison emphasizes the functional and evolutionary themes vertebrates carry within their structures. Comparison also helps formulate the questions we might ask of structure. For example, different fishes have different tail shapes. In the homocercal tail, both lobes are equal in size, making the tail symmetrical (fig. 1-1.a). In the heterocercal tail, found in sharks and a few other groups, the upper lobe is elongated (fig. 1-1.b). Why this difference? The homocercal tail is found in teleost fishes as—salmon, tuna, trout. These fishes have a swim bladder, an air-filled sac that gives their dense bodies neutral buoyancy. They neither sink to the bottom nor bob to the surface, so they need not struggle to keep their vertical position in the water. Sharks, however, lack swim bladders, and so tend to sink. The extended lobe of their heterocercal tail provides lift during swimming to help counteract this sinking tendency. So, the differences in structure, homocercal versus heterocercal, are related to differences in function. Most students of biology are interested in man, but a study a man alone would lead to an extremely narrow conception of his place on the earth. In order properly to understand the position of man in the world of life, a knowledge of his relations to other living things is almost essential. Man is classified as a member of a large group of animals called the vertebrates. They are organisms which have a backbone , or vertebral column, and include such diverse creatures as snakes, fishes, birds, frogs, elephants, and mice.
Figure 1.1: Homocercal and heterocercal fish tails. Form differs because function differs. (a) Sweeping, side-to-side movements of the homocercal tail, common in fishes with neutral buoyancy, drive the body forward. (b) Swimming strokes of the heterocercal tail propel the fish forward, and motion of the long extended upper lobe imparts an upward lift to the posterior end of the fish. Sharks, which are a good deal denser than water, need the upward forces provided by the extended lobe of the tail to counteract a tendency to sink Chordate Characteristics The differences among the chordate taxa are more apparent than the similarities that unite them. Most vertebrates have an endoskeleton, a system of rigid internal elements of bone or cartilage beneath the skin. Some vertebrates are terrestrial, and most use jaws to feed on big food particles. But protochordates are all marine animals, none are terrestrial, and all lack a bony or cartilaginous skeleton. However, their support system may involve rods of collagenous material. All taxa of chordates, despite these superficial differences, share a common body design similar in at least five fundamental features: notochord, pharyngeal slits, endostyle or thyroid gland, dorsal hollow nerve cord, and postanal tail (fig. 1-2)
Figure 1- 2: Generalized chordate characteristics. A single stream of water enters the chordate mouth, flows into the pharynx, and then exits through several pharyngeal slits. In many lower chordates, water exiting through the slits enters the atrium, a common enclosing chamber, before returning to the environment via the single atriopore.The endostyle, not shown, is a food-groove that runs along the floor of the pharynx. These five features diagnose the chordates, and taken together, distinguish them from all other taxa. 1-Notochord The notochord is a slender rod that develops from the mesoderm in all chordates. It lies dorsal to the coelom but beneath and parallel to the central nervous system (brain and spinal cord). The phylum takes the name Chordata from this structure. Typically, the notochord is composed of a core of cells and fluid encased in a sheath of fibrous tissue (fig. 1-3). mechanical structures, in which the outer wall encloses a fluid core, are called hydrostatic organs. The notochord is a hydrostatic organ with elastic properties that resist axial compression. It lies along the body axis to allow lateral flexion but prevents collapse of the body during locomotion.
Figure 1- 3: Cross section of the notochord of a frog tadpole. 2-Pharyngeal slits The pharynx is a part of the digestive tract located immediately posterior to the mouth. During some point in the lifetime of all chordates, the walls of the pharynx are pierced, or nearly pierced, by a longitudinal series of openings, the pharyngeal slits. In vertebrates, gills form adjacent to these pharyngeal slits. The slits are openings only, often with no significant role in respiration. In many primitive chordates, these openings serve primarily in feeding, but in embryos they play no respiratory role; therefore gill slits is a misleading term. Pharyngeal slits may appear early in embryonic development and persist into the adult stage, or they may be overgrown and disappear before the young chordate is born or hatched. Whatever their eventual embryonic or adult fate, all chordates show evidence of pharyngeal slits at some time in their lives 3- Endostyle or Thyroid Gland The endostyle is a glandular groove in the floor of the pharynx. The thyroid gland is an endocrine gland that produces two major hormones. The thyroid gland, like the endostyle, arises embryologically from the floor of the pharynx. The thyroid gland, like the endostyle, is involved in iodine metabolism, further suggesting a homology between the two, with the endostyle, the phylogenetic predecessor of the thyroid. chordates as (urochordates, cephalochordates, larval lamprey) have endostyles, and (adult lamprey, all other vertebrates) have thyroids. 4- Dorsal hollow nerve cord A third chordate characteristic is a dorsal hollow nerve cord derived from ectoderm, lie above the gut. The major nerve cord in most invertebrates is ventral in position, below the gut, and solid. In chordates, however, the nerve cord lies above the gut and is hollow along its entire length; or more accurately, it surrounds the neurocoel, a fluid-filled central canal (fig. 1-4). Figure 1-4: Dorsal hollow nerve cord. (a) Basic body plan of an annelid or arthropod. In such animals, a definitive nerve cord, when present, is ventral in position, solid, and lies below the digestive tract. (b) Basic chordate body plan.The nerve cord of chordates lies in a dorsal position above the digestive tract and notochord. Its core is hollow, or more correctly, it has a fluidfilled central canal, the neurocoel, indicated as the white spot in the dorsal hollow nerve cord. 5-Postanal tail chordates possess a postanal tail 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.
Biogenetic Law Pharyngeal slits, numerous branchial arches, and other fish characteristics even appear in the early embryos of reptiles, birds, and mammals, but they are lost as these tetrapod embryos proceed to term (fig. 1-5). These and many similar structures are remnants of fish features from the evolutionary past. from ovum to complete body, the individual passes through a series of developmental stages that are brief, condensed repetitions of stages through which its successive ancestors evolved. The biogenetic law states that ontogeny in abbreviated form recapitulates (repeats) phylogeny . Figure 1-5: Principles of embryology. (a) Preservationism. Across the early embryos, general features are preserved such as gill slits, tail, early limbs. But as embryonic development proceeds, it proceeds to the specific where the particular features of the adult to be are now established. Note, for instance, the changes in the snake and the bat. (b) Human embryology. Note that the embryo does not become first a tiny fish, followed by an amphibian, reptile (or bird), before becoming human.There is no embryonic recapitulation adult ancestors of during human development.Approximate embryo age in weeks is given beneath each. Homology and analogy A similar structure found in two or more organisms may have formed either from the same embryonic tissues in each organism or from different embryonic tissues. A structure that arises from the same embryonic tissues in two or more organisms sharing a common ancestor is said to be homologous. Even though the limb bones may differ in size, and some may be reduced or fused, these bones of the forelimb and hindlimb of amphibians, diapsids, and mammals are homologous to their counterparts (Fig. 1-6.a). The wings of insects and bats, however, are said to be analogous to one another (Fig. 1- 6.b). Although they resemble each other superficially and are used for the same purpose (flying), the flight surfaces and internal anatomy have different embryological origins. The forelimbs of sharks, penguins, and porpoises provide examples of convergent evolution.When organisms that are not closely related become more similar in one or more characters because of independent adaptation to similar environmental situations, they are said to have undergone convergent evolution, and the phenomenon is called convergence
Figure 1-6: (a) Homology: hindlimbs of a hawk, a salamander, a plesiosaur, an alligator, and an elk. Bones with the same intensity of shading are homologous, although they are modified in size and in details of shape by reduction or, even, fusion of bones (as in the elk and the hawk). Identical structures have been modified by natural selection to serve the needs of quite different animals. (b) Analogy: wings of an insect, a bird, a bat, and a pterosaur. In each, the flight surfaces and internal anatomy have different embryological origins; thus, the resemblances are only superficial and are not based on common ancestry or embryonic origin. Figure 1-6: (a) Homology: hindlimbs of a hawk, a salamander, a plesiosaur, an alligator, and an elk. Bones with the same intensity of shading are homologous, although they are modified in size and in details of shape by reduction or, even, fusion of bones (as in the elk and the hawk). Identical structures have been modified by natural selection to serve the needs of quite different animals. (b) Analogy: wings of an insect, a bird, a bat, and a pterosaur. In each, the flight surfaces and internal anatomy have different embryological origins; thus, the resemblances are only superficial and are not based on common ancestry or embryonic origin. Questions 1- Define the terms homology and analogy 2- compare between the structure and functions of heterocercal fins and homocercal fins 3- true or false a- protochordates are all marine animals b- In chordates, the nerve cord lies above the gut
Dr. Karim R. Hamad