APPENDAGES OF THE TRUNK. *77 



present is greater, in proportion as the animal is more elevated 

 in the scale. Thus we find them so modified as to become 

 antenna, those long, horn -like filaments, with which the head 

 is furnished in Insects and Crustacea, or to be subservient to 

 mastication by being converted into jaws, or to take the form of 

 legs, swimming organs, &c. Of this metamorphosis we shall 

 hereafter notice some remarkable examples in the class Crus- 

 tacea. Sometimes, the appendages of the dorsal arch are 

 present throughout, and perform, like those of the ventral arch, 

 the functions of limbs ; of this we find many examples amongst 

 the Annelida. But in general, no more than two pairs pre- 

 sent themselves ; these are situated on the segments constituting 

 the centre of the body ; and they perform the functions of wings 

 or of analogous organs ; as we shall hereafter see, when speak- 

 ing of the class of Insects. The legs are generally 6, 8, 10, or 

 14 in number ; sometimes many hundreds may be counted ; and 

 sometimes they are altogether deficient ; but when they exist at 

 all, they are never fewer than six, which is the number that is 

 characteristic of the class of I nsects. Sometimes instead of 

 distinct legs, we meet with strong bristly appendages, as in the 

 Earth-worm ; or bundles of such bristles, in the midst of which 

 one is occasionally a cirrhus or tendril-like appendage, constitut- 

 ing a sort of rudimentary leg, as in many Annelida (Fig. 388.) 



652. The tendency to'repetition ex- 

 hibited by the segments of the body, 

 is as remarkable in the disposition of 

 the muscles and of the nervous system, 

 as it is in the arrangement of the general 

 envelope. In<nost animals of this Sub- 

 kingdom, each ring in its complete state 

 possesses a pair of nervous ganglia, 

 united on the central line ; and these 

 ganglia are connected together by a 

 double cord of communication, which 

 runs along the lower or ventral surface 

 of the body. In the inferior Articulata, Fro. SBDNERVOUS SYSTEM o* 

 and even in the highest, previously to 

 the completion of their development, these ganglia are nearly 



