FREE-LIVING FRESH-WATER NEMATODES 



39 



Oncholaimus punctatus Spilophora canadensis 



Plectus tubifer Teratocephalus cornutus 



Prismatolaimus stenurus Trilobus longus (Leidy), Ba'stian 



Rhabditis punctatus Tripyla lata 



Rhabdolaimus minor Tylcnchus symmetricus 



There has been little time or opportunity to search specially for 

 these fresh water forms, and the fact that it has been possible in so 

 short a time to assemble so many genera and species is proof of 

 the enormous abundance and multifarious nature of these organ- 

 isms. No doubt there is a horde of species awaiting discovery in 

 North American waters. 



THE NEMATODE FORMULA. 



The following diagram illustrates the nature of the formula 

 used in the tabulation of the various necessary measurements : 

 =?#; 



Fig. 1. Diagram of the descriptive decimal formula used for nematodes; 6, 7, 8, 10, 

 6 are the transverse measurements, while 7, 14, 28, 50, 88 are the corresponding longi- 

 tudinal measurements. The formula in this case is: 



7. 14. 28. SO. 88. 



6. 7. 8. 10. 6. 



The unit of measurement is the hundreth part of the length 

 of the body, whatever that may be. The measurements become, 

 therefore, percentages of the length. The absolute length is given 

 in millimeters as a final non-paired term. This decimal method of 

 presenting the measurements is the most compact and convenient 

 method, and is being used more and more year by year. 



The measurements are taken with the animal viewed in profile ; 

 the first are taken at the base of the pharynx, the second at the 

 nerve-ring, the third at the cardiac constriction or end of the neck, 

 the fourth at the vulva in females and at the middle (M) in males, 

 the fifth at the anus. The formulae represent the average of sev- 

 eral specimens whenever possible. 



There are several methods of securing the measurements, one 

 of the most convenient being the preparation of a camera lucida 

 "skeleton" to scale, consisting of a line following the body axis 



