364 



Mr Cobb, On Free-living fresh-water 



Otherwise than in these few small points these descriptions 

 appear to add nothing of particular value to our knowledge of the 

 Nematode group. 



Formula used in the descriptions. 



Fig. 1 illustrates the formula used in the text to express 

 the necessary measurements. The formula derived from the 



Fig. 1. Diagram in explanation of the descriptive formula used for Nematode 

 worms. 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- 

 6- 7- 8- 10- 6-' 



The unit of measurement is the hundredth part of the length of the worm, 

 whatever that may be. The measurements become, therefore, percentages of the 

 length. 



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. 



diagram is however capable of considerable amplification, for by 

 simple modifications the fourth term above the line can be made 

 to express both the size and form of the internal sexual organs. 

 Thus, letting — (a dash) represent an outstretched, and ' (an apo- 

 strophe or quotation mark) a refiexed or contorted, ovary or testicle, 

 we have in the case of a male : — 



— M for a simple outstretched apparatus extending forward. 

 = M for a double outstretched apparatus extending forward. 



— M — for outstretched testicles extending in opposite directions. 

 ' — M for two testicles extending forward, one of them refiexed. 

 " M for two testicles extending forward, both refiexed, or 



contorted. Similarly in the case of a female, in which for the 

 sake of illustration we will place the vulva at 66 per cent, of the 

 length from the head : — 



' 66 ' for two refiexed ovaries extending in opposite directions. 



" 66 for two refiexed ovaries extending forward. 



— 66 — for two ovaries outstretched in opposite directions. 

 ' 66 for a single refiexed ovary extending forward. 



— 66 for a single outstretched ovary extending forward. 

 66 — for a single outstretched ovary extending backward. 

 66 ' for a single refiexed ovary extending backward. 



