Homologus organs
Homology is a similarity of structure in various organisms arising from common ancestry and is usually reflected in common embryological origin.Such structures called homologus organs are the consequences of divergent evolution and enable the organism to carried out different functions.
Superficially, they look different. the study of homologies is a major aspect of comparative anatomy. example of homologies in organs and organ system of all groups of living organisms.
These homologues provide important evidence for evolution. the forelimbs of vertebrates provide an excellent example of organ with similar structure but different function. The following of several vertebrates such as those of amphibians, Lizards, Birds, bats and men are all constructed on the same base plan and include similar bone in the same position.
But they have become morphologically different in the course of evolution as a result of modification to suit different requirements. Some examples of vertebrate forelimbs modified to suit various functions.
A special type of homology is shown by metameric animals. A striking example is that of the appendages of arthropods especially those of crustaceans.In the typical crustacean pair of appendages is borne by each segment of the body. it is appendages have evolved from a single structure plan and modify in a serial order so as to perform various function this is known as serial Homology. An Example of serial homology in the prawn.
Homology Represent the consequence of adaptive radiation from a common ancestor in every organism to adapt to different ecological niches.
Classical example of adaptive radiation is a structure of the following of mammals .there is always a single long bone the humerus in the upper arm.In the forum there are two parallel bones the Anna and the radius in the wrist they are typically 8 carpal bones are arranged as two rows of four.5 parallel metacarpals from The Skeleton of the palm of the hand and row of three phalanges each from skeleton of the digit expecting the first digit which has only two phalanges.
The tenrecs (scaly ant eater) of the order insectivora show the primitive pentadactyl and structure.The moles, which are at relatives are highly modified for digging in order to adapt a subterranean habitat.
All the bones of the limbs are short and broad and give the kimb a shovel like appearance. In the bats of the order Chiropetra, the humerus,radius and ulnaas well as four digits are greatly elongated so as to support the wing membrane. The forelimb is thus modified for flight .
examples of homologus structure in plants are, a thorn in a boungainvilla Or a rose and a tendril in the cucurbits, are homologus, arising as they do in the axillary position.
Analogous Organs
Analogous organs are morphologically different structures which develop in various unrelated organisms serving similar function. However, there are a certain similarities in these structure which are based on adaptation to perform the same function. These organs, called analogous organs, are the consequences of convergent evolution.
A classical example of analogus organs is the set of wings developed Independently by insects, some extinct reptiles as well as birds and bats. The insect wings is a Membrane supported by chitinous veins. The wings of pterosaurs, extinct flying reptiles, we’re formed by a fold of skin supported by an enormously enlarged fourth digit of the forelimbs. In birds, the planning surface of the wing is composed of feathers.
The feathers are supported by an internal skeleton of bones of the forelimbs. In bats the wings is formed by a membrane modified from skin. The wing in the bat is supported by the elongated and outspread phalanges of the last four digits of the forelimbs.
In plants examples are Tendrils. The tendrils of pea and vine have different origin. The former is a modified leaf and the latter is a modified stem.
Vestigial Organ
Another very important aspects of comparative anatomy is the study of vestigial or rudimentary organ. The presence of vestigial organs further strengthens our notion of speciation through evolution. The vestigial represents structure that were once useful to the ancestors and in course of evolution got modified in the descendents. Such modifications during evolutions are produced as a result of adaptations to changed environmental conditions. In many cases the reduced organs has assumed a new function , completely unlike that for which it was originally adapted. In other cases, it is apparently useless.
Most widely known example of vestiges is the human vermiform appendix. This is a small structure, wormlike in the appearance (vermiform) and is the constricted terminal portion on the caecum of the large intestine. In some mammals that live on coarse fibrous diet, made up of a considerable amount of cellulose, the caecum and appendix are Considerably larger. This enables the bacteria lodged therein to enzymetiacally digest the cellulose. However, in humans with changed dietary habits (cellulose intake is much reduced) the appendix has been rendered superfluous and therefore, remains rudimentary. Sometimes it is the source of nuisancd because it’s enlargement result in appendicitis . In man, it can be removed by surgery without dire consequences.
There are numerous other example of vestigial organs in animals and plants. Nictitating membrane of the eye,the external ear muscle and the wisdom teeth in humans ‚the vestigial caudal vertebrae (embryonic tail) in higher primary the pelvic girdle in whale, Boas ‚pythons, degenerated eye in cave dwelling vertebrates and invertebrates are such examples in animals.
In plants some of the example of vestiges are covered stomata on the stem of cactus, vestigial leaves in prickly pears,the abortive stamens of labiatae and the abortive pistillodes of male flower in Cucurbitaceae.
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