116 ANNELIDA 



3. On the midline between these pigment spots are white 



spots. 



4. If the specimen is favorable you may find with a lens 

 a series of segmental sense organs on the first ring of each 

 somite. They are of unknown function, the eyes are supposed 

 to be developed from certain of these organs, and they are 

 landmarks in determining the morphological boundaries of 



somites. 



5. On the median line in the groove that separates the 

 most posterior ring from the sucker find the anus. 



Make a drawing of the dorsal surface. 

 On the ventral surface: 



1. The mouth, at the anterior end of the body, is bounded 

 by the proboscis dorsally and anteriorly and by the fourth 

 ring ventrally. Determine its shape. The mouth with the 

 region around it forms the anterior sucker. 



2. The male reproductive aperture is on the median ven- 

 tral portion of the thirtieth ring. This is surrounded by a 

 thickening. 



3. The female reproductive aperture is on the median line 

 at the posterior margin of the thirty-fifth ring. This is not 

 marked by a distinct thickening. 



4. Find four apertures with thickened margins arranged 

 in the form of a square between the thirty-ninth and fortieth 

 and the fortieth and forty-first rings. These are the aper- 

 tures of the mucous glands. 



5. Find the external apertures of the nephridia about half 

 way between the median line and the margins, on the pos- 

 terior edge of the last ring of a somite. The spots are light 

 colored and elliptical. These are important landmarks in 

 determining the boundaries of somites. Some of the anterior 

 and posterior somites do not bear them. 



6. Observe the size, shape, and structure of the posterior 

 sucker. 



