514 FRESH-WATER BIOLOGY 



surrounded by a row of granular cells but without muscle elements. 

 It does not terminate in a bulb though the end of the cell row may 

 be slightly enlarged. 



The esophagus opens directly into the following region which is 

 commonly termed the intestine. It is the digestive portion of the 

 canal and is without any cuticular lining. The cavity is of con- 

 siderable size and lined by large cells rich in protoplasm. This 

 region changes gradually into the narrow terminal section of the 

 canal, known in the female as the rectum, or in the male as the 

 cloaca, since the duct of the sex gland joins it to form a common 

 passage way. 



The tail is ordinarily sharply pointed though sometimes the 

 point is short and in other cases long drawn out. The anal open- 

 ing is ventral, a little anterior to the tip of the body. In a few 

 instances the anus is terminal and the tail is rounded or of peculiar 

 form. In several families its true character is obscured in the 

 male because lateral wings or folds of cuticula cover it. These 

 folds may be low, narrow, keel-like ridges along the sides or may 

 have developed into wide semi-circular wings forming together a 

 clasping organ known as the bursa. Protoplasmic strands in the 

 wings appear like ribs of an umbrella; they vary in form and 

 number and are much used in the diagnosis of species. Numerous 

 papillae occur on the ventral surface of the male both in front of 

 and behind the anus. They vary greatly in size and arrangement 

 in different species and constitute another useful feature in the 

 determination of genera and species. A prominent cup-shaped 

 sucker is found on the ventral surface in front of the anus in some 

 species and one can often see in the body behind the anal orifice a 

 few large unicellular structures which are interpreted as glands. 



Between the head and the tail there are very few external fea- 

 tures to be noted. A minute excretory pore lies in the mid- ventral 

 line not far from the middle of the esophagus. In the female the 

 sexual pore also is found on the ventral surface; in some families 

 it is near the head, in others near the tail, and again in the center of 

 the body. Its location is an important characteristic in defining 

 the various groups. 



A circumesophageal nerve ring with lateral ganglia is a conspic- 



