MACROBDELLA. 103 



On the dorsal surface notice : 



1. Near the anterior end a series of ten small black spots 

 arranged in the form of a horseshoe with the arched end for- 

 ward. These are the eyes. They are arranged in pairs on the 

 first, second, third, fifth, and eighth rings. These are believed 

 to be on the first five somites. The first and second somites 

 comprise a single ring each; the third includes the third and 

 fourth rings; the fourth, the fifth, sixth, and seventh rings; 

 the fifth, the eighth, ninth, and tenth rings. 



2. Near the lateral edges notice the black pigment spots. 

 The larger spots are situated mostly on a single ring, but may be 

 extended on to others. Smaller pigment spots may occur on 

 other rings along the same line. There is evidence that these 

 larger spots mark the anterior rings of each somite wherever they 

 occur. How many rings are commonly included in a somite? 



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 notice: 



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

 the proboscis dorsally and anteriorly and by the fourth ring ven- 

 trally . Determine its shape. The mouth with the region 

 around it forms a sucker. 



2. The male reproductive aperture on the median ventral 

 portion of the thirtieth ring. This is surrounded by a thicken- 

 ing. 



3. The female reproductive aperture on the median line at 

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

 by a distinct thickening. 



