1911 Encyclopædia Britannica/Larval Forms
LARVAL FORMS, in biology. As is explained in the article on Embryology (q.v.), development and life are coextensive, and it is impossible to point to any period in the life of an organism when the developmental changes cease. Nevertheless it is customary to speak of development as though it were confined to the early period of life, during which the important changes occur by which the uninucleated zygote acquires the form characteristic of the species. Using the word in this restricted sense, it is pointed out in the same article that the developmental period frequently presents two phases, the embryonic and the larval. During the embryonic phase the development occurs under protection, either within the egg envelopes, or within the maternal body, or in a brood pouch. At the end of this phase the young organism becomes free and uses, as a rule, its own mouth and digestive organs. If this happens before it has approximately acquired the adult form, it is called a larva (Lat. larva, ghost, spectre, mask), and the subsequent development by which the adult form is acquired constitutes the larval phase. In such forms the life-cycle is divided into three phases, the embryonic, the larval and the adult. The transition between the first two of these is always abrupt; whereas the second and third, except in cases in which a metamorphosis occurs (see Metamorphosis), graduate into one another, and it is not possible to say when the larval stage ends and the adult begins. This is only what would be expected when it is remembered that the developmental changes never cease. It might be held that the presence of functional reproductive organs, or the possibility of rapidly acquiring them, marks off the adult phase of life from the larval. But this test sometimes fails. In certain of the Ctenophora there is a double sexual life; the larva becomes sexually mature and lays eggs, which are fertilized and develop; it then loses its generative organs and develops into the adult, which again develops reproductive organs (dissogony; see Chun, Die Ctenophoren des Golfes von Neapel, 1880). In certain Amphibia the larva may develop sexual organs and breed (axolotl), but in this case (neoteny) it is doubtful whether further development may occur in the larva. A very similar phenomenon is found in certain insect larvae (Cecidomyia), but in this case ova alone are produced and develop parthenogenetically (paedogenesis). Again in certain Trematoda larval stages known as the sporocyst and redia produce ova which have the power of developing unfertilized; in this case the larva probably has not the power of continuing its development. It is very generally held by philosophers that the end of life is reproduction, and there is much to be said for this view; but, granting its truth, it is difficult to see why the capacity for reproduction should so generally be confined to the later stages of life. We know by more than one instance that it is possible for the larva to reproduce by sexual generation; why should not the phenomenon be more common? It is impossible in the present state of our knowledge to answer this question.
The conclusion, then, that we reach is that the larval phase of life graduates into the later phases, and that it is impossible to characterize it with precision, as we can the embryonic phase. Nevertheless great importance has been attached, in certain cases, to the forms assumed by the young organism when it breaks loose from its embryonic bonds. It has been widely held that the study of larvae is of greater importance in determining genetic affinity than the study of adults. What justification is there for this view? The phase of life, chosen for the ordinary anatomical and physiological studies and labelled as the adult phase, is merely one of the large number of stages of structure through which the organism passes during its free life. In animals with a well-marked larval phase, by far the greater number of the stages of structure are included in the larval period, for the developmental changes are more numerous and take place with greater rapidity at the beginning of life than in its later periods. As each of the larval stages is equal in value for the purposes of our study to the adult phase, it clearly follows that, if there is anything in the view that the anatomical study of organisms is of importance in determining their mutual relations, the study of the organism in its various larval stages must have a greater importance than the study of the single and arbitrarily selected stage of life called the adult.
The importance, then, of the study of larval forms is admitted, but before proceeding to it this question may be asked: What is the meaning of the larval phase? Obviously this is part of a larger problem: Why does an organism, as soon as it is established at the fertilization of the ovum, enter upon a cycle of transformations which never cease until death puts an end to them? It is impossible to give any other answer to this question than this, viz. that it is a property of living matter to react in a remarkable way to external forces without undergoing destruction. As is explained in Embryology, development consists of an orderly interaction between the organism and its environment. The action of the environment produces certain morphological changes in the organism. These changes enable the organism to move into a new environment, which in its turn produces further structural changes in the organism. These in their turn enable, indeed necessitate, the organism to move again into a new environment, and so the process continues until the end of the life-cycle. The essential condition of success in this process is that the organism should always shift into the environment to which its new structure is suited, any failure in this leading to impairment of the organism. In most cases the shifting of the environment is a very gradual process, and the morphological changes in connexion with each step of it are but slight. In some cases, however, jumps are made, and whenever such jumps occur we get the morphological phenomenon termed metamorphosis. It would be foreign to our purpose to consider this question further here, but before leaving it we may suggest, if we cannot answer, one further question. Has the duration and complexity of the life-cycle expanded or contracted since organisms first appeared on the earth? According to the current view, the life-cycle is continually being shortened at one end by the abbreviation of embryonic development and by the absorption of larval stages into the embryonic period, and lengthened at the other by the evolutionary creation of new adult phases. What was the condition of the earliest organisms? Had they the property of reacting to external forces to the same extent and in the same orderly manner that organisms have to-day?
For the purpose of obtaining light upon the genetic affinities of an organism, a larval stage has as much importance as has the adult stage. According to the current views of naturalists, which are largely a product of Darwinism, it has its counterpart, as has the adult stage, in the ancestral form from which the living organism has been derived by descent with modification. Just as the adult phase of the living form differs owing to evolutionary modification from the adult phase of the ancestor, so each larval phase will differ for the same reason from the corresponding larval phase in the ancestral life-history. Inasmuch as the organism is variable at every stage of its existence, and is exposed to the action of natural selection, there is no reason why it should escape modification at any stage. But, as the characters of the ancestor are unknown, it is impossible to ascertain what the modification has been, and the determination of which of the characters of its descendant (whether larval or adult) are new and which ancient must be conjectural. It has been customary of late years to distinguish in larvae those characters which are supposed to have been recently acquired as caenogenetic, the ancient characters being termed palingenetic. These terms, if they have any value, are applicable with equal force to adults, but they are cumbrous, and the absence of any satisfactory test which enables us to distinguish between a character which is ancestral and one which has been recently acquired renders their utility very doubtful. Just as the adult may be supposed, on evolution doctrine, to be derived from an ancestral adult, so the various larval stages may be supposed to have been derived from the corresponding larval stage of the hypothetical ancestor. If we admit organic evolution at all, we may perhaps go so far, but we are not in a position to go further, and to assert that each larval stage is representative of and, so to speak, derived from some adult stage in the remote past, when the organism progressed no further in its life-cycle than the stage of structure revealed by such a larval form. We may perhaps have a right to take up this position, but it is of no advantage to us to do so, because it leads us into the realm of pure fancy. Moreover, it assumes that an answer can be given to the question asked above—has the life-cycle of organisms contracted or expanded as the result of evolution? This question has not been satisfactorily answered. Indeed we may go further and say that naturalists have answered it in different ways according to the class of facts they were contemplating at the moment. If we are to consider larvae at all from the evolution point of view, we must treat them as being representative of ancestral larvae from which they have been derived by descent with modification; and we must leave open the question whether and to what extent the first organisms themselves passed through a complicated life-cycle.
From the above considerations it is not surprising to find that the larvae of different members of any group resemble each other to the same kind of degree as do the adults, and that the larvae of allied groups resemble one another more closely than do the larvae of remote groups, and finally that a study of larvae does in some cases reveal affinities which would not have been evident from a study of adults alone. Though it is impossible to give here an account of the larval forms of the animal kingdom, we may illustrate these points, which are facts of fundamental importance in the study of larvae, by a reference to specific cases.
The two great groups, Annelida and Mollusca, which by their adult structure present considerable affinity with one another, agree in possessing a very similar larval form, known as the trochosphere or trochophore.
A typical trochosphere larva (figs. 1, 2) possesses a small, transparent body divided into a large preoral lobe and a small postoral region. The mouth (4) is on the ventral surface at the junction of the preoral lobe with the hinder part of the body, and there is an anus (7) at the hind end. Connecting the two is a curved alimentary canal which is frequently divided into oesophagus, stomach and intestine. There is a preoral circlet of powerful cilia, called the “velum” (2), which encircles the body just anterior to the mouth and marks off the preoral lobe, and there is very generally a second ring of cilia immediately behind the mouth (3). At the anterior end of the preoral lobe is a nervous thickening of the ectoderm called the apical plate (1). This usually carries a tuft of long cilia or sensory hairs, and sometimes rudimentary visual organs. Mesoblastic bands are present, proceeding a short distance forwards from the anus on each side of the middle ventral line (6), and at the anterior end of each of these structures is a tube (5) which more or less branches internally and opens on the ventral surface. The branches of this tube end internally in peculiar cells containing a flame-shaped flagellum and floating in the so-called body cavity, into which, however, they do not open. These are the primitive kidneys. The body cavity, which is a space between the ectoderm and alimentary canal, is not lined by mesoderm and is traversed by a few muscular fibres. Such a larva is found, almost as described, in many Chaetopods (fig. 1), in Echiurus (fig. 2), in many Gastropods (fig. 3), and Lamellibranchiates (fig. 4). This typical structure of the larva is often departed from, and the molluscan trochosphere can be distinguished from the annelidan by the possession of a rudiment at least of the shell-gland and foot (figs. 3 and 4); but in all cases in which the young leaves the egg at an early stage of development it has a form which can be referred without much difficulty to the trochosphere type just described. A larva similar to the trochosphere in some features, particularly in possessing a preoral ring of cilia and an apical plate, is found in the Polyzoa, and in adult Rotifera, which latter, in their ciliary ring and excretory organs, present some resemblance to the trochosphere, and are sometimes described as permanent adult trochospheres. But in these phases the resemblance to the typical forms is not nearly so close as it is in the case of the larva of Annelida and Mollusca.
In the Echinodermata there are two distinct larval forms which cannot be brought into relation with one another. The one of these is found in the Asteroids, Ophiuroids, Echinoids and Holothuroids; the other in the Crinoids.
The first is, in its most primitive form, a small transparent creature, with a mouth and anus and a postoral longitudinal ciliated band (fig. 5, A). In Asteroids the band of cilia becomes divided in such a way as to give rise to two bands, the one preoral, encircling the preoral lobe, and the other remaining postoral (fig. 5, B). In the other groups the band remains single and longitudinal. In all cases the edges of the body carrying the ciliary bands become sinuous (fig. 6) and sometimes prolonged into arms (figs. 7-9), and each of the four groups has its own type of larva. In Asteroids, in which the band divides, the larva is known as the bipinnaria (fig. 7); in Holothurians it is called the auricularia (fig. 6); in Echinoids and Ophiuroids, in which the arms are well marked, it is known as the pluteus, the echinopluteus (fig. 9) and ophiopluteus (fig. 8) respectively.
All these forms were obviously distinct but as obviously modifications of a common type and related to one another. They present certain remarkable structural features which differentiate them from other larval types except the tornaria larvae of the Enteropneusta. They possess an alimentary canal with a mouth and anus as does the trochosphere, but they differ altogether from that larva in having a diverticulum of the alimentary canal which gives rise to the coelom and to a considerable part of the mesoblast. Further, they are without an apical plate with its tuft of sensory hairs.
In Crinoids the type is different (fig. 10), and might belong to a different phylum. The body is opaque, and encircled by five ciliary bands, and is without either mouth, anus or arms, and there is a tuft of cilia on the preoral lobe. A resemblance to the other Echinoderm larvae is found in the fact that coelomic diverticula of the enteron are present.
The larvae of two other groups present certain resemblances to the typical Echinoderm larvae. The one of these is the tornaria larva of the Enteropneusta (fig. 11), which recalls Echinoderms in the possession of two ciliary bands, the one preoral and the other postoral and partly longitudinal, and in the presence of gut diverticula which give rise to the coelom; but, like the trochosphere, it possesses an apical plate with sensory organs on the preoral lobe. The resemblance of the tornaria to the bipinnaria is so close that, taking into consideration certain additional resemblances in the arrangement of the coelomic vesicles which arise from the original gut diverticulum, it is impossible to resist the conclusion that there is affinity between the Echinoderm and Enteropneust phyla. Here we have a case like that of the Tunicata in which an affinity which is not evident from a study of the adult alone is revealed by a study of the young form. The other larva which recalls the Echinoderm type is the Actinotrocha of Phoronis (fig. 12), but the resemblance is not nearly so close, being confined to the presence of a postoral longitudinal band of cilia which is prolonged into arm-like processes.
|After J. Müller.||After J. Müller.|
|Fig. 7.—Bipinnaria elegans, the Larva of a Star-fish. Description and lettering as in fig. 6.||Fig. 8.—Ophiopluteus bimaculatus, the Larva of an Ophiurid. Description and lettering as in fig. 6.|
The following groups have larvae which cannot be related to other larvae: the Porifera, Coelenterata, Turbellaria and Nemertea, Brachiopoda, Myriapoda, Insecta, Crustacea, Tunicata. We may shortly notice the larvae of the two latter.
In the Crustacea the larvae are highly peculiar and share, in a striking manner, certain of the important features of specialization presented by the adult, viz. the presence of a strong cuticle and of articulated appendages and the absence of cilia. They are remarkable among larvae for the number of stages which they pass through in attaining the adult state. However numerous these may be, they almost always have, when first set free from the egg, one of two forms, that of the nauplius (fig. 13, A) or that of the zoaea (fig. 13, B). The nauplius is found throughout the group and is the more important of the two; the zoaea is confined to the higher members, in some of which it merely forms a stage through which the larva, hatched as a nauplius, passes in its gradual development. The nauplius larva is of classic interest because its occurrence has enabled zoologists to determine with precision the position in the animal kingdom of a group, the Cirripedia, which was placed by the illustrious Cuvier among the Mollusca.
In the Tunicata the remarkable tadpole larva, the structure and development of which was first elucidated by the great Russian naturalist, A. Kowalevsky, possesses a similar interest to that of the nauplius larva of Cirripeds, and of the tornaria larva of the Enteropneusta, in that it pointed the way to the recognition of the affinities of the Tunicata, affinities which were entirely unsuspected till they were revealed by a study of the larvae.
With regard to the occurrence of larvae, three general statements may be made. (1) They are always associated with a small egg in which the amount of food yolk is not sufficient to enable the animal to complete its development in the embryonic state. (2) A free-swimming larva is usually found in cases in which the adult is attached to foreign objects. (3) A larval stage is, as a rule, associated with internal parasitism of the adult. The object gained by the occurrence of a larva in the two last cases is to enable the species to distribute itself over as wide an area as possible. It may further be asserted that land and fresh-water animals develop without a larval stage much more frequently than marine forms. This is probably partly due to the fact that the conditions of land and fresh-water life are not so favourable for the spread of a species over a wide area by means of simply-organized larvae as are those of marine life, and partly to the fact that, in the case of fresh-water forms at any rate, a feebly-swimming larva would be in danger of being swept out to sea by currents.
1. The association of larvae with small eggs. This is a true statement as far as it goes, but in some cases small eggs do not give rise to larvae, some special form of nutriment being provided by the parent, e.g. Mammalia, in which there is a uterine nutrition by means of a placenta; some Gastropoda (e.g. Helix waltoni, Bulimus), in which, though the ovum is not specially large, it floats in a large quantity of albumen at the expense of which the development is completed; some Lamellibranchiata (Cyclas, &c.), Echinodermata (many Ophiurids, &c.), &c., in which development takes place in a brood pouch. In the majority of cases, however, in which there is a small amount of food yolk and no special arrangements for parental care, a larva is formed. No better group than the Mollusca can be taken to illustrate this point, for in them we find every kind of development from the completely embryonic development of the Cephalopoda, with their large heavily-yolked eggs, to the development of most marine Lamellibranchiata and many Gastropoda, in which the embryonic period is short and there is a long larval development. The Mollusca are further specially interesting for showing very clearly cases in which, though the young are born or hatched fully developed, the larval stages are passed through in the egg, and the larval organs (e.g. velum) are developed but without function (e.g. Paludina, Cyclas, Onchidium). As already mentioned, the larval form of the Mollusca is the trochosphere.
2. Free-swimming larvae are usually formed when the adult is fixed. We need only refer to the cases of the Cirripedia with their well-marked nauplius and cypris larvae, to Phoronis with its remarkable actinotrocha, to the Crinoidea, Polyzoa, &c. There are a few exceptions to this rule, e.g. the Molgulidae amongst the fixed Tunicata, Tubularia, Myriothela, &c., among the Hydrozoa.
3. Internal parasites generally have a stage which may be called larval, in which they are transferred either by active or passive migration to a new host. In most Nematoda, some Cestoda, and in Trematoda this larva leads a free life; but in some nematodes (Trichina) and some cestodes the larva does not become free.
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