EXTERNAL ANATOMY OF THE EARTHWORM 1 



1. Examine an Earthworm and note the long, tubular body which is 

 composed of a large number of segments. Identify the anterior, poste- 

 rior, dorsal, and ventral regions of the body. Count the segments of 

 your specimen and compare the number with others at your table. Note : 

 (a) the mouth, which is situated at the anterior end of the body below 

 the projecting lobe (prostomium) ; (b) the anus situated at the posterior 

 end of the body, where it can be seen as a vertical slit in the last segment, 

 and (c) the swollen region (clitellum) lying in the region between seg- 

 ments 32 and 37 in Lumbricus terrestris, a common species. 



2. With one hand take hold of the anterior end of the Earthworm and 

 draw the body through the fingers of your other hand. The rough feel- 

 ing is due to the presence of bristles (setae) which project through the 

 body wall. Ascertain the arrangement of the setae and the number 

 present on each segment. 



3. Examine your specimen with the dissecting microscope and find the 

 following openings : (a) openings of the sperm ducts in the swellings on 

 the ventral surface of segment 15 ; (6) openings of the oviducts just 

 lateral to the inner double row of setae in segment 14 ; (c) openings of 

 the seminal receptacles in the grooves between segments 9 and 10, 

 and 10 and 11, on a line with the outermost row of setae; (d) openings 

 of the nephridia just lateral and anterior to the setae of the inner row 

 on either side of each segment ; (e) openings of the dorsal pores on the 

 anterior end of each segment in the median dorsal line. 



4. Make (a) a drawing, twice natural size, of the anterior forty seg- 

 ments of the body from the ventral surface, and (b) a drawing, at the 

 same magnification, of the posterior ten segments of the body from the 

 dorsal surface to show the structure as observed. 



5. For comparison, examine a number of other species of Annelida, 

 such, for example, as a free-swimming type, Nereis (p. 83), or Autolytus 

 which reproduces by budding; a tube-dwelling type (Amphitrite, Areni- 

 cola, or Chaetopterus) ; and the medicinal Leech (Hirudo). (W. f. 45.) 



i B. pp. 81-84. 



279 



