ANNELIDA 



229 



as long as broad, and is usually cylindrical or flattened dorso- 

 ventrally. 



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

 ment and the muscular layers of the body wall. Being filled 

 with the body fluids it is a very important instrument of loco- 

 motion. This is accomplished by the alternate contractions of 

 the circular and longitudinal fibres with which the wall is sup- 

 plied. Externally there is a cuticula, usually very thin, over- 

 lying and secreted by the layer of hypodermal cells. Some of 

 the cells of the hypodermal layer are glandular and others are 

 sensory. The setse or bristles are secretions of the hypodermal 



Fig. 103. 



Fig. 103. Transverse section of Dero (diagrammatic). X300. c, coelom; c.^, cells of the 

 so-called "lateral line;" d.m., dermo-muscular wall including muscles and skin; d.v.^ dorsal blood 

 vessel; ect. ectoderm: ent, entoderm; g, gut; g.f.< giant nerve fibres; g/, glandular cells assisting in 

 digestion; m.c, circular muscle fibres; m, longitudinal muscle fibres; «, nephridium; «.»., ventral 

 nerve chain, made up of nerve cells and nerve fibres; s, setae; v.v., ventral blood vessel. 



Questions on the figure. — Compare this with Pig. 102 and identify all the struc- 

 tures which appear in both. What elements enter into the dermo-muscular wall? 

 Identify nerve cells, fibres and the "giant fibres" in the ventral nerve cord. 



cells and lie in sacs in the integument. These structures 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 



