TI1K CLASS <>l-' JN SECTS. 



Arthropodoaa 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 hfliifi'i-ii/ xi/inweti'y. In a less 

 marked degree there is ;ds<> :m utitcro- 

 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 pails 

 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. 

 Fig .2* In nearly all Worms (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, Flgi 3> 

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



* Professor Wyman (On Symmetry and Homology in Limbs, Proceeding.-; of the 

 Boston Society of Natural History, ISiiT) lias shown thai antero-posterior symmetry 

 is very marked in Articulates. In the adjoining liururc <>!' Jirm (Fig. 2) the longi- 

 tudinal lines illustrate what is meant by bilateral symmetry, and the traiisver.-e. 

 lines "fore and aft" symmetry. The two aatero-posterior halrM of the body are 

 very symmetrical in the Crustacean genera .In rn, oni-n-n.^, l'rri'//i<>. and other 

 Crustacea, and also among the Myriopods, S<-iiti;i< r/i, r i/;/(/cs;;/(/s. in which the 

 limbs are repeated oppositely, though with different decrees of inequality, from Uie 

 centre of the body backwards and forwards." "Lcnckart and Van Bcneden have 

 shown that Mysis has an ear in the last >egment.and Schmidt lias described an eye 

 in the same part in a worm, Aiitiiliicoru." /'/ iryninn. 



Flu. :! represent^ an ideal section of a Worm, /indicates tin 1 >kin, or mus- 

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

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



