228 



ZOOLOGY 



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

 vent rally. 



265. 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 contractions 

 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 



FIG. 103. 



c I 



FIG. 103. Transverse section of Dero. X 300. c., ccelom; c.L, cells of the so-called "lateral 

 line;" d.m., dermo- muscular wall including muscles and skin; d.v., dorsal bloodvessel; *, ecto- 

 derm; ent, entoderm; g, gut; g. /., giant nerve fibres; gl, glandular cells assisting in digestion; m.c., 

 circular muscle fibres; m., longitudinal muscle fibres; n, nephridium; n.v., ventral nerve chain, 

 made up of nerve cells and nerve fibres; 5, setae; v.v., ventral blood vessel. 



Questions on the figure. Compare this with Fig. 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 skin. These structures vary in num- 

 ber 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 



