THE CLASS OF INSECTS. 



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 antero- 

 posterior 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 (Fig. 2), and the single parts 

 on the median line of the body corre- 

 spond. Thus the labrum and clypeus 

 are represented by the tergite of the 

 eleventh segment of the abdomen. 

 Flg .2. In all Articulates (Fig. 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. b 

 The 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 membran- 

 ous expansions of the legs ; or, 

 as in the Insects (Fig. 4), of delicate tubes (tracheae), which 



* Professor Wyman (On Symmetry and Homology in Limbs, Proceedings of the 

 Boston Society of Natural History, 1867) has shown that antero-posterior symmetry 

 is very marked in Articulates. In the adjoining figure of Jcera (Fig. 2) the longi- 

 tudinal 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 the Myriapods, 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 MjrtSis has an ear in the last segment, and Schmidt has described an eye 

 in the same part in a worm, Amphicora." From Wyman. 



FIG. 3 represents an ideal section of a Worm. / indicates the skin, or mus- 

 cular body-wall, which on each side is produced into one or more fleshy tubercles, 

 usually tipped with bristles or hairs, which serve as organs of locomotion, and 



