223 



c 



J 



FIG. 101. Transverse section of Dero. x 300. 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; 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 nerve fibres; s, 

 setae; i\v. t ventral blood vessel. 



Questions on the figure. Compare this with Fig. 100 and identify 

 all the structures 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. 



260. Worms as a rule have no external skeleton other than 

 the cuticle, but in some instances a tubular protective structure 

 is formed by secretion or by cementing together small particles 

 of foreign matter. Because of the absence of hard skeletal 

 parts little is known concerning the worms of past geological 

 ages. 



261. Digestive System. The stomodaeum, the mesenteron, 

 arid proctodseum (see 90) are all to be distinguished in the 

 digestive canal. The mouth is not quite terminal, but slightly 

 ventral. The prostomium (or preoral lobe), a muscular ex- 

 tension of the oral segment, overarches it. There is typically 

 an enlarged muscular pharynx which is often eversible, fol- 

 lowed by a narrow tubular .oesophagus. Often there is no 



