222 ZOOLOGY. 



outside into rings as the name (Annulata) implies. The 

 internal organs also show metamerism, but in various ways. 

 These organs may pass directly through, with slight segmental 

 modification, as the digestive tube and ventral nerve cord; 

 they may be repeated independently in each segment, as the 

 setae or nephridial tubules ; or they may be represented in only 

 one or a limited number of segments, as the brain or the 

 reproductive bodies. The segments are not therefore exactly 

 equivalent, yet the agreement between successive segments is 

 sufficient to merit the term homonomous (see 121). The 

 number of segments varies from three to hundreds. The body 

 is from four or five to many times as long as broad, and is 

 usually cylindrical or flattened dorso-ventrally. 



259. The dermo-muscular sac is composed of the integu- 

 ment or skin and the muscular layers of the body wall. Being 

 filled with the body fluids it is a very important instrument 

 of locomotion. This is accomplished by the alternate con- 

 tractions of the circular and longitudinal fibres with which 

 the wall is supplied. Externally there is a cuticula, usually 

 very thin, overlying and secreted by the layer of epidermal 

 cells. Some of the cells of the epidermal layer are glandular 

 and others are sensory. The setae or bristles are secretions of 

 the epidermal cells and lie in sacs in the skin. These struc- 

 tures vary in number and position but are usually paired, 

 two or four groups to each segment. They are absent in the 

 leeches. Next to the skin is a layer of circular muscle fibres, 

 and within these are the longitudinal bands of muscle fibres. 

 In the leeches there are also dorso-ventral fibres. Special 

 groups of fibres occur in connection with the setae, the mouth 

 parts, suckers, etc. The fibres in worms are spindle-shaped 

 and unstriate. The dermo-muscular wall bounds a true body 

 cavity in the chaetopods; but in leeches the coelom is almost 

 filled with connective tissue. This suggests the condition in 

 many of the unsegmented worms. See Figs. 99, 101. 



