624 FRESH-WATER BIOLOGY 



tough thick shell, often covered with hooks and knobs, that seem 

 to anchor them in place. When the embryo has reached its full 

 development its vigorous movements burst the shell and a full- 

 grown animal emerges. 



No one has yet described a male and it is uncertain that any 

 other type than the female exists. This form may have a sperm- 

 producing organ yet undiscovered and thus be in reality hermaph- 

 roditic or may reproduce exclusively by parthenogenesis. 



The Gastrotricha have been studied but little in North America. 

 Of the seventy-five species thus far described only sixteen have 

 been recorded from the United States. Most of these were found 

 and described by Stokes at Trenton, New Jersey. While this dis- 

 tribution appears highly local there is no doubt that search in 

 other places will demonstrate for American species the same wide 

 general distribution that has been shown for European forms. 

 Further study will doubtless result also in the discovery of many 

 other species on this continent, since the evidence thus far secured 

 indicates that, like most minute aquatic organisms, these forms, too, 

 are cosmopolitan in distribution. 



KEY TO NORTH AMERICAN FRESH-WATER GASTROTRICHA 



1 (31) Caudal end prolonged into two prominent lateral processes. 



Suborder Euichthydina . . 2 

 Each caudal process encloses a glandular apparatus and bears a pore at the tip. 



2 (n) Body naked, scaled, or covered with rugosities or papillae, but never 



bearing spines Family ICHTHYDIDAE . . 3 



3 (6) Body without scales; cuticula smooth. 



Ichthydium Ehrenbeig 1830 . . 4 

 Seven or eight species described; only two reported from North America. 



4 (5) Surface of body entirely smooth, no constant furrows or ridges. 



Ichthydium podura O. F. Miiller 1786. 



Total length 0.075 mm.; esophagus 0.0188 mm., caudal process 0.00875 mm. 

 long. Breadth of anterior region 0.0163 mm. The cuticula is thin and is 

 often laid in deep wrinkles, but these are entirely inconstant. The lateral 

 margins show no trace of being flattened. The caudal processes are sharply 

 set off from the body. Eggs smooth. 



FlG. 965. Ichthydium podura in dorsal view. X 360. (After Zelinka.) 



