OR OSSEOUS SYSTKM. 33 



of shorter setse placed below. This lateral organ of motion 

 is seen on a more magnified scale in Fig. 14.3, where a re- 

 presents the jointed cirrhus, and b,c the tuft of setae. These 

 segments and their appendices vary much in their degree of 

 consolidation, in some being soft and transparent, and in others 

 opaque dense and covered with pearly or metaUic lustre. The 

 tubicolous annelides which form the densest exterior tubes 

 have generally the segments less marked, and covered with a 

 softer skin than those which are found in soft tubes, or are en- 

 tirely without such protection. We have thus already developed 

 in the annelides, the segments of the body and their jointed 

 tubular appendices for progressive mo- FIG. 15. 



tion, and all the essential parts of the 

 skeleton which present themselves 

 under various forms throughout the 

 entomoid classes. 



X. Myriapoda. In the myriapods 

 the skeleton is more dense, the articula- 

 tions are more distinct, and the jointed 

 tubular appendices, for progressive mo- 

 tion, are more developed from the sides 

 of the segments than in the helminthoid 

 classes. Their muscles having firmer 

 points of attachment, act with more 

 energy and effect, and their movements 

 effected in the thinner medium of the air 

 are more lively than those of the hel- 

 minthoid animals which mostly inhabit 

 a more dense aquatic medium. The seg- 

 ments of the body are here more numer- 

 ous thanin the higher articulated classes, 

 and they are still, as in the annelides, 

 nearly equally developed throughout the 

 whole trunk. In some, as the iuli, the 

 segments are calcareous, hard, and cylin- 

 drical, and the lateral appendices or feet 

 are short, and for the most part double 

 on each side of each segment. In other 

 myriapods, as the scolopendra, repre- 

 sented in Fig. 15, the segments are de- 

 pressed, coriaceous, composed simply 



PART I. 



