or Cypris stage are discernible [fig. 5 (5), (6)] beneath the cuticle. When this is moulted the free-swimming cypris larva is liberated with its six pairs of biramous thoracic legs, its bivalve 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 foreign 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.
 After C. Claus, Schriften der Gesellsch. |
|  After C. Claus, Untersuch. zur Erforschung in optical section. | |
| Fig. 6.—Cypris larva of Lepas fascicularis. |
(1) first antenna; | (6) tergum; | |
| (1) first antenna; | (2) compound eye; | (7) biramous feet; | |
| (2) compound eye; | (3) liver; | (8) carina; | |
| (3) simple eye; | (4) simple eye; | (9) cement gland. | |
| (4) biramous appendages. | (5) scutum; | ||
These changes lead to the attainment of the adult form and structure. When they are completed the cuticle, including the shell-valves, is cast off and the young cirriped emerges. This is the second and final metamorphosis, which resembles in its main features the metamorphosis of the metabolous Insecta.
Metamorphosis occurs in most groups of the animal kingdom. It is generally found in attached organisms, for these nearly always have free-swimming larvae and the metamorphosis occurs when the change of habit is effected. For the details of the process the reader is referred to systematic works on zoology. Here only the most striking instances of it can be mentioned. It occurs in a remarkable form in some sponges, in which at the metamorphosis the larval epidermis, which acts as a locomotive organ, is said to become transformed into the collared flagellated cells of the canal system, the adult epidermis being a new formation. It occurs in the 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 peculiar; the brain and sense organs of the larva pass into the stomach and are digested. In the 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 Amphioxus there is no metamorphosis though the larval changes are most remarkable and extensive, but the larval life is a long one and the development very gradual, the new organs coming into function as soon as they are formed.
(After J. Müller.)
Fig. 8.—A ventral view of a bipinnaria carrying the body of the young star-fish.
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 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 absorbed 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 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, oesophagus and anus are new formations, and the central nervous system of the larva when present shares the fate of the larval locomotory apparatus. In Asteroids and Crinoids the metamorphosis is accompanied by fixation to foreign objects, the fixation being effected as in Cirripedes by the preoral lobe.
In the Vertebrata a metamorphosis occurs in the lamprey and the 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 anything 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 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 internal organs undergo disruption and are reformed. It happens also in those nemertine 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 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. Se.*)
METAPHOR (Gr. μεταφορά, transfer of sense, from μεταφέρειν, to carry over), a figure of speech, which consists in the transference to one object of an attribute or name which strictly and literally is not applicable to it, but only figuratively and by analogy. It is thus in essence an emphatic comparison, which if expressed formally is a “simile” (Lat. similis, like); thus it is a metaphorical expression to speak of a ship ploughing her way through the waves, but a simile when it takes the form of “the ship, like a plough, moves,” &c. The “simple” metaphor, such as the instance given, becomes the “continued” metaphor when the analogy or similitude is worked out in a series of phrases and expressions based on the primary metaphor; it is in such “continued metaphors” that the solecism of “mixed” metaphors is likely to occur.
METAPHYSICS, or Metaphysic (from Gr. μετά, after, φυσικά, things of nature, φύσις, i.e. the natural universe), the accepted
name of one of the four great departments of philosophy (q.v.).
The term was first applied to one of the treatises of Aristotle
on the basis of the arrangement of the Aristotelian canon made
by Andronicus of Rhodes, in which it was placed “after the
physical treatises” with the description τὰ μετὰ τὰ φυσικά. The
term was used not in the modern sense of above or transcending
nature (a sense which μετά cannot bear), but simply to convey
the idea that the treatise so-called comes “after” the physical treatises.[1] It is therefore nothing more than a literary accident that the term has been applied to that department or discipline of philosophy which deals with first principles. Aristotle himself described the subject matter of the treatise as “First
