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METAMORPHOSIS
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METAMORPHOSIS , a See also:term used in See also:zoology in different senses by different authors, and sometimes in different senses by the same author. E. Korschelt and K. Heider, in their See also:work on the development of the Invertebrata, usually apply it to the whole of the larval development. For instance, in their See also:account of the Bryozoa, they say (p. 18, See also:part 2, of the See also:English See also:translation) : " The metamorphosis of a Bryozoan larva comprises a more or less protracted See also:free-See also:swimming See also:stage during which no perceptible advance is made in the development of the larva, and the subsequent somewhat complicated changes which bring about its transformation into the first See also:primary zooid of the See also:young Bryozoan See also:colony." Throughout their account of the See also:Crustacea they use the word in the same sense, i.e. as applied to the whole of the changes which the larva undergoes in passing into the adult. On the other See also:hand, in their account of See also:Mollusca they seem to restrict the term to the final See also:change by which the larva passes into the adult See also:form (op. cit., part 4, p. 14). F. See also:Balfour in his See also:great work on See also:Comparative See also:Embryology seems to limit the word to a sudden change in the larval See also:history. For instance, he says: " The See also:chief point of See also:interest in the above development is the fact of the See also:primitive nauplius form becoming gradually converted without any See also:special metamorphosis into the adult See also:condition " (Comparative Embryology, 1885, i. 463). " By the free Cypris stage into which the larva next passes a very See also:complete metamorphosis has been effected " (op. cit. i. 490). " The change under-gone by the See also:Tadpole in its passage into the See also:Frog is so considerable as to deserve the name of a metamorphosis " (op. cit. ii. 137). Finally and most decisively he says in his See also:general account of larvae: " In the larval type [of development] they are See also:born at an earlier stage of development, in a condition differing to a greater or less extent from the adult, and reach the adult See also:state either by a See also:series of small steps or by a more or less considerable metamorphosis " (op. cit. ii. 36o). See also:Mere the term will be used in the sense of the last quotations from Balfour and will be regarded as applicable only to those cases of sudden and marked change which frequently occur at the end of the larval See also:period and sometimes at more or less frequent intervals during its course (Crustacea). Some authors (see H. G. See also:Bronn, Thierreich, " See also:Myriapoda," Bd. 5, Abth. 2, p. 113) have applied the term " metamorphosis',-" only to those cases of larval development in which the young leaves the See also:egg with provisional See also:organs which are lost in the later development. Such authors apply the term " See also:anamorphosis " to cases in which the just-hatched young is without provisional organs but differs from the adult in See also:size, and in the number of segments and See also:joints, &c. Such writers apply the term " epimorphosis " when there is merely an acquisition of sexual maturity and increase in size after See also:birth or hatching. The essential feature of metamorphosis is the sudden bursting into See also:function of new organs, whether these organs suddenly arise or have been gradually formed, without becoming functional in preceding larval stages. ' Another feature of it is the disappearance of organs which have been of use to the larva but which are not required at all or are not required in the same form in the new environment. The term is only used in connexion with larval development and is not applied to the sudden changes, due to a change of environment (e.g. the passage of the mammalian embryo from the oviduct into the uterus), which sometimes occur in embryos. Neither is it used in connexion with the sudderi changes of conditions which occur at the birth or hatching of an embryo, although, especially,,. in the See also:case of birth, this event is frequently accompanied by profound morphological alteration. The most See also:familiar examples of metamorphosis are the abrupt changes which occur at the end of the larval history of the frog and of many See also:insects. ' In both these cases there is a sudden and great change of environment; there is a sudden demand for new organs which would have been quite useless in the old environment, and organs which were of use in the old environment and are of no use in the new have to be eliminated. The two examples we have chosen have the See also:advantage of showing us the two methods by which the crisis in the See also:life-history is met. In the frog (fig. r) the structural changes which obtain full fruition at the metamorphosis take See also:place gradually during the previous tadpole life. They relate mainly to the alterations of the See also:respiratory organs and vascular See also:system which are required for the purely terrestrial life of the frog, and to the See also:appearance of the paired limbs. The changes in the respiratory and vascular (Rana temporaria). A, See also:Side view of an advanced tadpole with well-See also:developed posterior limbs; the anterior limbs are See also:present but hidden beneath the operculum. B, Ventral view of the same with operculum removed showing the anterior limbs in situ; the ventral See also:body See also:wall has also been removed and the See also:heart (ht) and See also:intestine exposed. (br) Gills; (KL) spiracle. C, A frog after the metamorphosis but before the absorption of the tail. organs are led up to in the tadpole, which during the greater part of its aquatic life is a truly amphibious See also:animal, breathing by lungs as well as by gills; but a sudden change occurs in these organs at the metamorphosis. The limbs which were slowly formed during tadpole life—the posterior pair visibly, the anterior under See also:cover of the operculum (fig. s, B)—are of no use to the tadpole and must constitute a pure See also:burden to it. The See also:principal events of the metamorphosis are the sudden appearance of the anterior limbs, and the complete See also:closure of the gill See also:aperture (fig. I, C). The appearance of the anterior limbs and the acquisition of functional importance by both pairs enable the frog to leave the See also:water and pass on to the See also:land to See also:lead its terrestrial See also:fife. The other larval organs, such as the gills and the tail, gradually shrink in size and ultimately vanish. In the case of the gills this shrinkage had begun before the See also:meta-morphosis, but the tail shows no sign of diminution until the frog is ready to pass on to theland. The distinguishing feature of this type of metamorphosis is that the animal is burdened for a certain period, both before and after, with organs which are useless to it. In the next type, which is exemplified by the metabolous Insecta, this occurs to a much smaller extent, although the changes of See also:habitat and the corresponding changes of structure are more remarkable. In insecta the change is usually from a terrestrial or aquatic habitat to an aerial one. The larva of a butterfly is a See also:worm-like organism which creeps on and voraciously devours the foliage of certain See also:plants (fig. 2, C). During its life it undergoes much growth, but no important change in structure. When it leaves the egg After Leuckart and Nitsche's Wanrltafeln, by See also:mission of T. G. See also:Fisher & Co. brassicae, L. A, Imago (See also:female), side view. B, Pupa fixed by a See also:cord across the See also:middle of the body and by the tail. C, See also:Caterpillar. (I) Forewing; (5) thoracic legs; (2) antenna; (6) See also:hind wing; (g) labial palp; (7) the See also:head; (4) first maxilla; (8) the See also:thorax; (9) the See also:abdomen, some of the segments of which in the caterpillar carry a pair The imaginal disks ap- of prolegs (to). See also:pear to arise in the embryo in which they develop, some of them from the epiblast and some from the hypoblast. They persist practically unchanged through larval life and become active as centres of growth in the pupa. The pupal stage in such a metamorphosis may be compared to a second embryonic stage in which the organs of the adult assume their final shape. In this See also:kind of metamorphosis the larval organs are entirely got rid of in the pupal stage, during which the See also:insect is as a See also:rule incapable of locomotion and takes no See also:food; and the new formation of organs—especially those of locomotion and alimentation—which is necessitated by the totally different habits of the larva and mature insect, is also accomplished at the same period, largely, no doubt, at the expense of the material afforded by the disrupted larval organs. The larva itself does not form any of these organs and carry them about during its active life, though it does possess the very See also:minute centres of growth known as the imaginal disks which burst into activity after the larval life is over. It must not be sup-posed that in all insects in which the sexual animal has a different habitat from the young form, there is a metamorphosis of the kind just described. In the may-flies and See also:dragon-flies, in which the larva is aquatic, the change is prepared for some See also:time before the actual metamorphosis, the organs which See also:ate,. necessary for the aerial existence being gradually acquired during larval life. In such cases, the metamorphosis belongs to our first type and consists of the See also:act by which the organs previously and gradually acquired suddenly become functional. We have now considered in detail two typical cases of meta-morphosis. In the first the change is gradually led up to and the larva is burdened, in its later stages at least, with organs which are of no use to it and only become functional at the metamorphosis. In the other, the change is not led up to. It is sudden, and a kind of second embryonic period is established After Leuckart and Nitsche's Wandtafeln, by permission of T G. Fisher & Co. it is adapted to live and feed on a particular See also:species of plant, on or near which the eggs are deposited by the See also:parent butterfly. It has powerful biting jaws by which it procures its See also:vegetable food. The adult, on the other hand, is a winged creature which also lives on plants but in quite a different way to the larva (fig. 2, A). It flies from plant to plant and obtains its food by sucking the juices of. See also:flowers and other parts. The powerful mandibles of the larva have disappeared and in their place we find a suctorial See also:proboscis formed by the first maxillae [fig. 2, A (4)1. Between the larva and the adult insect there is interposed a resting stage, the so-called pupa (fig. 2, B), during which no food is taken, but very important changes of structure occur. These changes consist of two processes: (I) histolysis, by which most of the larval organs are destroyed by the See also:action of phagocytes; and (2) histogenesis, by which the corresponding organs of the imago are developed from the imaginal disks.
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to enable the important and far-reaching transformation to be accomplished. It is clear that the two kinds of metamorphosis
A and B after Fritz See also: But this is in our See also:opinion an incorrect use of the word. The typical larval development of a Crustacean consists of a series of small metamorphoses. At each See also:moult new organs which have been developed since the preceding moult become See also:manifest and some of them functional. For instance, the See also:prawn Penaeus leaves the egg as a nauplius larva (fig. 3, A). It issues from the first moult as a metanauplius which has a forked tail, a beginning of the cephalo-thoracic See also:shield, and a large See also:helmet-shaped upper See also:lip. It also possesses stump-like rudiments of the maxillae and two anterior pairs of maxillipeds. After the next moult it is known as a protozoaea (fig. 3, B), in which a cephalo-thoracic shield is well developed, the posterior part of the body is prolonged into a tail, in the anterior part of which the thoracic segments are obscurely indicated, and the four pairs of stump-like rudiments have become functional appendages [fig. 3, B (4), (5), (6), (7)1 This passes into a later protozoaea stage (C) in which the rudiments of the See also:compound eyes and of the abdominal segments are visible beneath the cuticle and in which certain functional changes (jointing, &c.) have appeared in the limbs. This is succeeded by the zoaea stage (fig. 3, D), characterized by the stalked and functional condition of the eyes, the increased size of the abdominal segments, and the appearance of appendages on the sixth of them, the increase of size in the third pair of maxillipeds (8) which had appeared as small rudiments in the preceding stage, and the appearance of the five pairs of posterior thoracic limbs as small biramous appendages. The zoaea stage is followed by the mysis stage (fig. 3, E) in which the thoracic feet are biramous, as in Mysis. From this the adult form proceeds. The transformation is more See also:gradual than would be gathered from this See also:short description, because moults (7) upper lip; (8) frontal sense organs. occur during the later stages from each of which the larva comes with some slight transformation. In the life-history of a typical Cirripede there may be said to be two distinct metamorphoses, with gradual developmental stages taking place between them. The animal is hatched as a nauplius. This undergoes a series of moults during which increase in size and slight changes in form occur (fig. 4, A, B). At the last of them several organs characteristic of the second thorax; abdomen ; See also:liver; frontal sense See also:organ, just be-hind which are the compound eyes; (at) to (a6) the six abdominal appendages. (9) (tto) (II) (i2) After See also:Spence Bate in See also:Annals and See also:Magazine of Nat. History, vol. 8, and series, 1851. A, As just hatched; B, After the first moult. (i) first pair of nauplius appendages; (2) second (3) third „ 4) upper lip; 5) frontal sense organ. Aftei C. Claus, Untersuch. zur Erforschung Crustaceen-Systems. (i) first antenna; (2) second , (3) mandibles; (4) rudiment of the maxilla; (5) first pair of biramous limbs (6) sixth 2 24 See also:METAPHOR= or Cypris stage are discernible [fig. 5 (5), (6)] beneath the cuticle. When this is moult&d the free-swimming cypris larva is liberated with its six pairs of biramous thoracic legs, its bivalve See also:shell, and its paired compound eyes (fig. 6). This is the first metamorphosis. After a certain period of free life the Cypris larva attaches itself by its anterior antennae to some See also:foreign See also:object and enters upon the pupal stage (fig. 7). During this the larva takes no food and ceases to move, and undergoes important changes of structure and form beneath the larval cuticle, which invests it like a pupal case. These changes lead to the After C. Claus, Untersuch. zur Refor- schung Crustaceen-Systems. in See also:optical See also:section. (i) first antenna; (6) tergum; (2) compound (7) biramous eye; feet; (3) liver; (8) carina; (4) simple eye; (9) See also:cement (5) scutum; gland. attainment of the adult form and structure. When they are completed the cuticle, including the shell-valves, is See also:cast off and the young cirriped emerges. This is the second and final metamorphosis, which resembles in its See also:main features the meta-morphosis of the metabolous Insecta. Metamorphosis occurs in most See also:groups of the animal See also:kingdom. It is generally found in attached organisms, for these nearly always have free-swimming larvae and the metamorphosis occurs when the change of See also:habit is effected. For the details of the See also:process the reader is referred to systematic See also:works on zoology. Here only the most striking instances of it can be mentioned. It occurs in a remarkable form in some See also:sponges, in which at the metamorphosis the larval epidermis, which acts as a See also:locomotive organ, is said to become transformed into the collared flagellated cells of the See also:canal system, the adult epidermis being a new formation. It occurs in the See also:Polyzoa, and is, in some of these, characterized by an almost complete disruption of the larval organs and a subsequent new formation of the organs of the adult. The metamorphosis in such cases belongs to our second type, the new organs being new formations at the metamorphosis and not developed from rudiments which make their appearance in the earlier larval history. In Phoronis the metamorphosis of the larva (Actinotrocha), which occurs on fixation, is gradually led up to, but the mode of destruction of some of the larval organs is See also:peculiar; the See also:brain and sense organs of the larva pass into the See also:stomach and are digested. In the See also:Tunicata, in which fixation of the free larva is effected by the head, as in Cirripedia and some, if not all, Polyzoa, the metamorphosis occurs entirely after fixation as a rapid series of developmental changes which occur ad hoc and are not prepared for by preceding changes. In See also:Amphioxus there is no metamorphosis though the larval changes are most remarkable and extensive, but the larval life is a See also:long one and the development very gradual, the new organs coming into function as soon as they are formed. In most Mollusca there is also a prolonged and important larval life, marked by very interesting stages of structure (trochosphere, veliger, &c.), but it is not usual to speak of a metamorphosis for the changes are gradual, each organ developing with great rapidity and coming into function at once. In certain forms, however, a metamorphosis occurs, e.g in the glochidium larva of Anodonta, which embeds itself in the skin of a See also:fish and there metamorphoses into the adult. In the Echinodermata there is a particular stage in the larval history, when the ciliary locomotive apparatus breaks up and is ab- sorbed and the animal takes to its creeping adult life. This metamorphosis is gradually prepared for in the precedent larval development by changes which ultimately lead to the complete See also:establishment of the adult radial symmetry. The metamorphosis belongs therefore to our first type, but it is remarkable for the heavy burden of adult structures which the larva, in its later stages at least, carries about (fig. 8). The adult body is, in the main, fashioned out of the larval body, and it takes over most of the organs of the latter; but as a rule the adult mouth, See also:oesophagus and anus are new formations, and the central See also:nervous system of the larva when present shares the See also:fate of the larval locomotory apparatus. In Asteroids and Crinoids the metamorphosis is accompanied by fixation to foreign See also:objects, the fixation being effected as in Cirripedes by the preoral See also:lobe. In the See also:Vertebrata a metamorphosis occurs in the See also:lamprey and the See also:Amphibia. The metamorphosis of the lamprey is peculiar. It lives for three or four years as a sexless larva, known as the ammocoete. It then quite rapidly (in three or four days) undergoes a series of changes and becomes converted into the adult. The metamorphosis affects the alimentary canal, the eyes, the respiratory apparatus and other organs, and especially the reproductive organs, which become mature. The adult lives for a few months only, spawning soon after the metamorphosis. This metamorphosis belongs to our second type, but there does not appear to be any resting stage during the few days in which it is effected. In the Amphibia the metamorphosis is fairly exemplified by that of the frog. In many fishes there is a considerable larval development, but this is perfectly gradual and there does not appear to be any-thing of the nature of a metamorphosis. In most cases of metamorphosis those organs of the larva, which are found also in the adult, persist through the transformation, undergoing merely the See also:ordinary modifications of development. But it sometimes happens that such organs are completely destroyed and rebuilt during the metamorphosis. This is conspicuously the case in the metabolous Insecta, in some of which all the See also:internal organs undergo disruption and are reformed. It happens also in those nemertine See also:worms which develop by a larva; in these the larval epidermis is cast off, a new one having been formed. It is possible that the same phenomenon occurs in sponges. In most Echinoderms a similar phenomenon is observed with regard to the oesophagus and the mouth and anus. The probable explanation of this remarkable phenomenon would appear to be that in certain cases the larval organs become so highly specialized in connexion with the larval life that they are unable to undergo further change; new formation is therefore necessary. The phenomenon is one of considerable interest, for it is found in the case of the blastopore, in cases in which there is no metamorphosis, sometimes even in embryonic development. There can be little doubt that the mouth and anus are both genetically connected with the earlier blastopore and that the blastopore is homologous in most animals; and yet how seldom does the blastopore become transformed into the adult openings and how various is its fate. The See also:hypothesis suggested above applies-completely to this behaviour of the blastopore; that is to say, it is suggested that the primitive mouth or blastopore becomes, or has become in some vanished larval history, so highly specialized in connexion with larval needs that it is unable to give rise to both mouth .and anus, and in some cases to either. (A. Additional information and CommentsThere are no comments yet for this article.
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