LARVA, the an animal, when it differs from its parents in form and manner of life. In most invertebrates and in some of the lower vertebrates, the animal hatched from the egg is so different from the adult that in many cases the relationship was long unsuspected by naturalists, and the little creatures were given names as separate beings,— wee', nautilus, etc., now applied to the forms of larva they repre sent. These larvae may grow by imperceptible degrees into the stature and likeness of the adult; or they may pass bx, comparatively sud den changes through a series of more or less •dIfferent forms, until finally the adult form is reached and retained. In the latter case the development is said to be by metamorphosis .(q.v.), most completely and familiarly mani fested by insects. Whatever the method, the •course of larval growth in its successive stages recalls the phylogeny of its race — that is, the course of its evolution in history. Thus each of the various phases of the larval life of any of known as the veliger may appear. This is characterized by a large disk on the top of the head, which serves for a time as a swimming organ and is later lost.
All of the lower and some of the higher crustacea pass through a so-called nauphusstage (Fig. 3). The adult crustacean consists of several segments, but the nauplius is without joints, has a single eve, a straight alimentary canal, the mouth being overhung by ;It enor mous upper lip, and three pairs of appendages, which later became changed into the 'two pairs of antenna; and the mandibles of the adult. The first pair of the nauplian appendages are the lower animals, like the foetal life of em bryos of the higher Tanks, indicates probable ancestral forms. Some of the most remark able larva may be mentioned. Among the marine annelids a larva known as the trocho or trochosphere (Fig. 1) is common. It ias a short compact body, traversed by the alimentary canal, and has one or more bands of cilia around the body and a sensory patch at the top of the head. By feeding, this larva grows, the increase being chiefly in length, and -with this increase, the joining or metamerism of the body, so noticeable in the adult, appears.
Other worms have different types of larvae, among them the pilidium of the nemertines (Fig. 2), shaped somewhat like a chapeau with enormous ear lappets. Between these is the mouth which leads to a large blind sac, the simple and are only sensory, while the two remaining pairs are two-branched, and serve as swimming organs, the basal portions being also used as jaws to force food into the mouth which lies between them. In the higher crustacea two other and better developed larva known as the zoia and megalops may appear.
Possibly the most remarkable larva occur among the echinoderms. These forms, exem plified by the starfish and sea-urchin, are no ticeable for their radial symmetry, but in the larva, of which there are several distinct types, not a trace of a radial arrangement of parts can be found. They are rather markedly bi laterally symmetrical, with well-marked dorsal and ventral surfaces, which, however, do not correspond with the upper and lower surfaces stomach of the worm. The worm itself de velops inside the pilidium and later escapes from it to continue its existence, leaving the rest to die.
Among the mollusks larva like the trocha phore occur, and it is the existence of these larva which leads naturalists to think that annelids and mollusks, so different in the adult, had a common ancestry. Later, with the appearance of molluscan characters, a larva of the adult. Some of these larva are more or less barrel-shaped, but in others, as in the plateus (Fig. 4) and bipinstaria (Fig. 5), the body is drawn out into a number of processes, soft and flexible in the latter, but stiffened in the pinteus by internal calcareous rods. The starfish or sea-urchin later arises on one tide. of this larva, the processes are absorbed and the radial arrangement is superimposed upon the bilatetal features in the adult, without, however, completely obliterating them.