THE WONDERS OF LIFE 



bristle-worms (polychceta) one or two pairs of short 

 unjointed feet (parapodia) are found in every joint. 



The other two chief classes of the articulates develop 

 long and jointed feet of very varied forms, and at the 

 same time assume different shapes of hmbs in the divi- 

 sion of labor. This heterogeneous articulation (heter- 

 onomy) is the more pronounced the higher the whole 

 organization. This is equally true of the aquatic, gill- 

 breathing Crustacea (crabs, etc.) and the tracheata 

 (terrestrial animals breathing through a trachea, the 

 myriopods, spiders, and insects). In the higher groups 

 of both classes the number of limbs is usually not high- 

 er than fifteen to twenty ; and they are distributed in 

 three principal sections — head, breast, and posterior part 

 of the body. The firm covering of chitine, which was deU- 

 cate and thin in most of the annelids, is much thicker in 

 most of the Crustacea and tracheata, and often hardened 

 by a calcareous deposit; it forms a solid ring of chitine in 

 each segment, inside which the motor muscles are at- 

 tached. The successive hard rings are connected by 

 thin, mobile, intermediate rings, so that the whole body 

 combines firmness, elasticity, and mobility in a high 

 degree. The structure of the long jointed legs, which 

 are fixed in pairs on each segment, is very similar. 

 Hence the typical character of the motor organs of the 

 Crustacea lies in the circumstance that both in the body 

 and the limbs the muscles are attached to the interior 

 of hollow chitine tubes, and go in these from member 

 to member. 



The vertebrates are just the reverse in structure. In 

 their case a solid internal skeleton is formed in the 

 longitudinal axis of the body, and the muscles are ex- 

 ternal to these supporting organs. The articulation or 

 metamerism itself is not visible externally in the verte- 

 brates; it is only seen in the muscular system when the 

 non-articulated skin has been removed. Then, even in 



2&Z 



