BY N. A. COBB. 455 



and 1'7 and '8 are the lengths of the corresponding diameters. 

 It will be seen that the diffei'ent dimensions are taken up in 

 the formula in a natural order. Reading the formula from 

 left to risht reads off the dimensions of the animal from head 

 to tail. Now comes the peculiarity of the formula : The tonit 

 of measurement is not absolute hut relative, is, in fact, nothing 

 else than the hundredth part of the length of the worm 

 itself. In other words, the measurements are expressed as 

 percentages of the total length of the animal. Thus, the first 

 measurement (1-) indicates that the depth, i.e., the length, of 

 the pharynx is equal to 1 per cent, of the total length of the 

 body. So the measurements for the base of the neck indicate 

 that the length of the neck is equal to 17 3 per cent, of the body- 

 length, while the length of the diameter at the base of the neck 

 is equal to 1'6 percent of the body-length. The absolute length 

 of the animal expressed in millimeters is placed at the right. In 

 the present case that length is r77 millimetei's.* In the formula 

 for males the measurements relating to the vulva of the female 

 are replaced by measurements relating to the middle of the body. 

 The fourth number above the line becomes, therefore, always 50, 

 and is indicated by M. The number below M gives the diameter 

 of the male at the middle. In other respects the formula for the 

 male is similar to that for the female. 



Unless otherwise stated all measurements must be understood 

 to be taken from adult specimens as they appear in profile. 



By the use of certain signs the fourth term above the line may 

 be made to convey an idea of the form and size, as well as the 



* Dividing 177 millimeters by one hundred, we obtain the unit of length 

 used in the remainder of the formula. The result is '0177 millimeters. If 

 it is desired to obtain the absolute length of the pharynx, neck, or other 

 part, multiply the proper measurement by this coefhcient (•0177). This 

 gives for the absolute length of the pharynx "0177 x 1— -0177 millimeters, 

 and for the length of the neck '0877 x 17'3 = '3 millimeters. It is well 

 known, however, that absolute dimensions, in such a case, are of very little 

 consequence, while relative dimensions are of the greatest importance, and 

 the great advantage possessed by such a formula as that now under consi- 

 deration is that relative dimensions are made prominent. 



