BODY PLAN OF THE EARTHWORM 1 



1. Examine, with the low and high power, a prepared transverse section 

 through the body of a small Earthworm. Draw the section in outline, 

 about four inches in diameter, and then fill in a quadrant to show the 

 general structure as follows : 



(a) Body Wall, which consists of: (i) a very thin, transparent outer 

 membrane (cuticle) ; (ii) a layer of elongated, epidermal cells (epi- 

 dermis) ; (Hi) a layer of circular muscles and (iv) a thicker layer of 

 longitudinal muscles, which together comprise most of the body wall ; 

 and (v) a thin layer of peritoneal epithelium lining the coelom. 

 Note (vi) the bristle-like setae, with attached muscle fibers, projecting 

 through the body wall. Determine their number and arrangement and 

 compare with paragraph 2, page 279. 



(b) Alimentary Canal, which appears in a transverse section as a 

 ring of tissue with a dorsal infolding which forms the typhlosole. The 

 wall of the canal consists of (i) an outer layer of cylindrical cells 

 (chloragogen layer) which also fill the cavity of the typhlosole ; (ii) a 

 muscular layer with both circular and longitudinal fibers ; (Hi) a vascular 

 layer with many tiny blood vessels; and, finally, (iv) a layer of elongated 

 cells (lining epithelium) which form the inner lining of the tract and 

 are the essential agents in the digestion and absorption of food. A sec- 

 tion through the dorsal and through the ventral blood vessels can be 

 noted, lying above and below the digestive tract respectively. 



(c) Nerve Cord, which consists of two distinct structures, the outer 

 envelope, or sheath, and the inner nervous portion. The sheath con- 

 sists of a thin, outer epithelial layer which encloses a thicker supporting 

 layer. Embedded in the supporting layer, dorsally, are the three giant 

 firers which run the length of the cord and, ventrally, the subneural 

 blood vessel. The nervous portion of the cord within the sheath is bilobed 

 and contains nerve cells and fibers. If the section happens to be through 

 the region of a ganglion a pair of the lateral nerves may be seen. 



1 B. pp. 84-86. 



285 



