In the Nereis, Aphrodite, and other erratic worms, external appendages become developed from the lateral aspects of the different seg ments, in the shape of bundles of seta', moved by muscles appropriated to each set, and these constitute the first rudiments of locomotive limbs. The senses are at the same time im proved in their condition, tentacula or feelers are found appended to the head, and the eyes become larger and more conspicuous, although still presenting the form of simple specks or ocelli.
In the MYRIAPODA the limbs become arti culated, and of sufficient strength to permit of a terrestrial existence, each one of the numerous legs having a distinct set of muscles appro priated to its movements, in addition to the muscular apparatus, whereby the segments of the as yet flexible and elongated body are endowed with motion, and which of course represent the strata of the muscular covering of the Leech, strengthened and endowed with the capability of more precise action, in proportion as the cuticular skeleton has become more dense and distinctly jointed. Antenna: are, moreover, now placed upon the head resem bling those of Insects, no doubt constituting organs of sense of a similar nature, and the eyes in Scolopendne are found to be very dis tinct and perfectly organized, but still only ocelli resembling the simple eyes of Insects.
In the admirably constructed class of IN SECTS, creatures adapted to an aerial existence, and consequently requiring the utmost exertion of muscular power to sustain their bodies in the air, the muscular system of the locomotive articulated appendages, the legs and wings, must undergo a still further improvement, and the means whereby this is accomplished are sufficiently manifest. The nervous ganglia are accumulated into a few large and powerful centres of innervation ; the rings of the body, to which the locomotive organs are attached, are dilated and strengthened in proportion to the force of the muscles placed within, and constitute three thoracic rings of such firmness and inflexibility that they may well be looked upon as forming a distinct division of the exo skeleton, and give rise to the distinction laid down by entomologists betweeen the head, the thoracic, and the abdominal segments that enter into the composition of an insect's body. But the same concentration of the nervous system, which permits an Insect to possess the extraor dinary powers of flight with which it is gifted, allows by the increased perfection given to the brain, the possession of elaborately con structed senses. The eyes assume a complexity
of structure that is truly wonderful, the sense of touch attains extreme delicacy, and indubita bly the means of smelling, of tasting, and of hearing are now conferred, however incapable we may be of pointing out the mode in which they are exercised ; nay, it is extremely proba ble that capabilities of perception of which we can form no idea, are bestowed upon the Insect races commensurate with the activity of their movements and the wide range of duties they are appointed to perform.
During the metamorphosis to which Insects are subject, that is to say during the advance ment of these creatures from an embryo condi tion to their mature or perfect state, changes are constantly in progress, both in the nature and arrangement of the locomotive organs, and of course as these changes are effected, the entire disposition and even the vital properties of the muscular system appropriated to their movements undergo a considerable modification. The larva; of many genera have externally the appearance of the simplest worm, being pro vided with not even any vestiges of the loco motive apparatus that subsequently is to be developed : even the rings or segments of the body are entirely soft, the cuticular covering being of extreme tenuity, and the tegumentary muscles, as a natural consequence, propor tionally rudimentary in their structure. In such larvae the nervous system exhibits the lowest condition found among the apodous An nelidans, and the eyes and external senses are, if they exist at all, of the humblest possible character. This is the case, for example, in the maggots of many Dipterous and Hymenop terous Insects.
In others, as for instance in the caterpillars of the Lepidoptera, the locomotive powers are of a slightly ameliorated description : the larva possesses a distinct head, and to the succeeding segments, rudely constructed limbs named legs, and others bearing still less resemblance to the locomotive members of the future insect, to which the name of pro-legs has been appro priated, are the only instruments of progression. Even in the most perfect larva?, as in those of the aquatic Beetles, the form is elongated and resembles that of an Annelidan ; the legs are comparatively feeble and of small size, and simple ocellt replace the compound eyes that afterwards become developed in the perfect Insect.