478 FRESH-WATER BIOLOGY 



double organ in the females only of some species of Oncholaimus , 

 located in the posterior part of the body and connecting with the 

 exterior through openings in the subdorsal region; (2) the gland- 

 like pair of organs seen in the females of Diplogaster, and apparently 

 also of Rhabditis and other related genera; and (3) the long-necked 

 paired glands sometimes emptying into the male cloaca. It is 

 conceivable that some of these serve a sexual function, such as 

 the secretion of a substance whose odor or taste is of service in 

 enabling the nematodes to locate their mates. 



88> 



FIG. 772. Diagram in explanation of the descriptive formula used for nematodes; 6, 7, 8, 10, 6 are 

 the transverse measurements, while 7, 14, 28, 50, 88 are the corresponding longitudinal measurements. 

 The formula in this case is: 



7. 14. 28. 50. 88. 

 6i 7^ 8l 10! 6. 



The measurements are simply percentages of the length, and the formula, as printed in the key, may 

 be regarded as somewhat in the nature of a conventionalized sketch of the nematode with dimensions 

 attached. 



The measurements are taken with the animal viewed in profile; the first is taken at the base of the 

 pharynx, the second at the nerve-ring, the third at the cardiac constriction (base of the "neck"), the 

 fourth at the vulva in females and at the middle (M ) in males, the fifth at the anus. (After Cobb.) 



It seems reasonably clear that fresh-water nematodes have 

 marked seasonal development, at least in some species. Adults 

 of many species can be found at all times of the year. Freezing 

 does not necessarily kill them. Although the fresh-water nema- 

 todes are so widespread, and so abundant at all seasons, it is not 

 always easy to isolate them for examination without the use of 

 special methods. Few of these nematodes exceed two to three milli- 

 meters in length, and they are so slender and transparent as to make 

 it practically impossible to examine them without the aid of a lens. 



However, when special methods are employed they may easily 

 be collected. A few centigrams of mud or sand from a place where 

 nematodes are believed to exist is disseminated in a watch glass of 

 water, and the sediment examined carefully for the characteristic 

 wavy non-progressive motion exhibited by these little organisms. 

 When discovered, the specimens are captured with a fine-pointed 

 pipette or medicine dropper and ejected with a minimum of other 



