THE EARTHWORM 109 



II. INTERNAL STRUCTURE 



Exercise 3. — The Digestive Tract and Coelom. 



(a) Fasten the specimen in a dissecting pan, dorsal surface up, by 

 pinning through the first somite and again toward the posterior end. 

 Make an incision through the body wall just back of the clitellum and 

 on the mid-dorsal line. With fine scissors, cut toward the head end, 

 using great care to cut nothing except the body wall and to keep on 

 the dorsal mid-line. Spread out the edges of the cut body wall and 

 pin them apart temporarily. Understand from Fig. 53 and others that 



crop g/zzarcf 



' 20 



esophagus^ 



p/fari/nx. 



f^'^^^^ bucca/ neri/'e open/ng of septum 



ca^/6y oord ca/c/ferous g/anc/ 



Fig. 53. — The anterior end of an earthworm, as if cut in the median, 



longitudinal plane. 



(Redrawn with modifications from A. M. Marshall and C. H. Hurst, "Practical Zoology," 

 copyright, 1895, by John Murray, printed by permission.) 



may be available how the transverse partitions, or septa, which con- 

 nect the inner surface of the body wall and the outer surface of the 

 digestive tract, are related to the coelom, the tract, and the body wall, 

 and why they must be broken if the body wall is to be properly spread 

 and the digestive tract exposed. Then break the septa from their 

 attachment to the body wall and pin the wall flat in the dissecting 

 pan; slant the pins outward to give room for fingers and instruments 

 in working. Examine demonstration preparation or consult instructor 

 if not sure your specimen is being properly dissected, since the proper 

 execution of this initial step is very necessary for the success of the 

 study that follows. 



(b) You will now be able to see the brownish stomach-intestine 

 and, on its upper surface, the large dorsal blood vessel. Toward the 

 head, the gut becomes differentiated and is partly hidden by other 

 organs which will be indicated presently. The cavity between the body 

 wall and the digestive tract is the body cavity, or coelom. With the 

 handlens and a needle, observe and feel the edges of the septa which 



