198 ELEMENTARY ZOOLOGY. 



segment of the body has a small nerve-center of its own in this ventral 

 chain, from which the nerves pass to the muscles and other organs 

 in the segment. The snout and lips receive nerves from the brain. 

 The Body Cavity. Inside the body wall there is a roomy cavity 

 in which all the internal organs lie (Fig. 57, c). The long digestive 

 tube runs from one end to the other of this cavity. The cavity 

 is divided up into as many chambers as there are segments, by means 



tct 



FIG. 57. Transverse section of Dero. X$oo. c, ccelom; d.m., the body wall, 

 consisting of skin and muscles; d.v., dorsal blood vessel; ect, ectoderm; ent, entoderm; 

 g, gut; g.f., large nerve fibres; gl, glandular cells assisting in digestion; m.c., circular 

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

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



Questions on the Figure. Compare this with Fig. 56 and identify 

 all the structures that appear in both. What work can the longi- 

 tudinal and circular muscle fibres do? What motions have the 

 setae? How produced? 



of thin circular partitions (Fig. 56, d) that run from the wall to 

 the digestive tract. The cavity contains a fluid similar to the blood. 

 The Digestive Tube. How nearly does it fill the body cavity? 

 How many different regions can you make out, either from dissec- 

 tion or from figures in the texts? In what particulars do they differ? 

 What names are given to them by the anatomists? What is the 

 function of each region? Does the tube show any signs of segmen- 

 tation corresponding to that outside? 



