Guide to the Study of Insects and a Treatise on Those Injurious and Beneficial to Crops - Couverture rigide
Présentation de l'éditeur
Articulated animals are also very distinctly bilateral, i.e. the body is symmetrically divided into two lateral halves, and not only the trunk but the limbs also show this bilateral symmetry. In a less marked degree there is also an anteroposterior symmetry, i.e. each end of the body is opposed, just as each side of the body is, to the other. The line separating the two ends is, however, imaginary and vague. The antennae, on the anterior pole, or head, are represented by the caudal, or anal, stylets (F ig. 2), and the single parts on the median line of the body correspond. Thus the labrum and clypeus are represented by the tergite of the eleventh segment of the abdomen. Flg.2. In all Articulates (F ig. 3) the long, tubular, alimentary canal occupies the centre of the body ;above it lies the heart, or dorsal vessel, and below, upon the under side, rests the nervous system. bT he breathing apparatus, or lungs, in Worms consists of |simple filaments, placed on the front of the head ;or of gill-like processes, as in the Crustaceans, which are formed by membranous expansions of the legs ;or, as in the Insects (F ig. 4), of delicate tubes (tracheae), which Professor Wyman (O nS ymmetry and Homology in Limbs, Proceedings of theB oston Society of Natural History, 1867) has shown that antero-posterior symmetry is very marked in A rticulates. In the adjoining figure of Jcera (F ig. 2) the longitudinal lines illustrate what is meant by bilateral symmetry, and the transverse lines fore and aft symmetry. The two antero-posterior halves of the body are very symmetrical in the Crustacean genera Jcera, Oniscus, Porcellio, and other Crustacea, and also among theM yriapods, Scutigera, Polydesmus, in which the limbs are repeated oppositely, though with different degrees of inequality, from the centre of the body backwards and forwards. Leuckart and Van Beneden have shown that
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