A Nematode Formula. 



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 per cent, 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 1"77 millimeters.* In the formula for males the 

 measurements relating to the vulva of the female are replaced by measure- 

 ments 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 position, of the sexual 

 organs. The female genital organs lie either on one or both sides of the 

 vulva, and the branches are either straight or reflexed. Letting a hyphen 

 represent a straight, and a quotation mark a reflexed branch, w^e have — 



-52"1- indicates two straight branches, one on eifclier side of the vulva. 



'52' 1' indicates two reflexed bi-anches, one on either side of the vulva. 



52" 1- indicates one straight branch behind the vulva. 



52'1' indicates one reflexed branch behind the vulva. 



-52*1 indicates one straight branch in front of the vulva. 



'52'1 indicates one reflexed branch in front of tlie vulva. 



In case of the male — 



-M- inditates two straight testicles extending in opposite directions. 

 -M indicates one straight testicle extending forwards. 



M- indicates one straight testicle extending backwards. 



M' indicates one reflexed testicle extending backwards. 

 'M indicates one reflexed testicle extending forwards. 



It should be borne in mind that the marks in the case of the male refer 

 ■ exclusively to the form and position of the testicle proper, that is to say, that 

 portion of the generative apparatus beyond the vas deferens. When two 

 testicles are present their limits are easily defined, for the point where they 

 join marks the beginning of the vas deferens. AVhen but a single testicle 

 exists it is marked of£ from the vas deferens by a constriction in the same 

 way that the vas deferens is marked ofE from the ductus ejaculatoi^us. 



The percentage of the body occupied by the sexual organs is indicated by 

 superior or reference figures placed at the right of and above the fourth 

 term. Thus '50'^^ represents a female sexual apparatus whose vulva is 

 central, and whose symmetrically reflexed branches occupy 30% of the 

 length of the body. 



* Dividing 1'77 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 pharvnx, neck, or other part, multiply the proper measiu'ement by 

 this co-eflicient (-0177) . This gives for the absolute length of the pharynx -0177 X 1 = '0177 

 millimeters, and for the length of the neck '01^7 X 17 3 =: -3 millimeters. It is well 

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

 wliile relative dimensions are of the greatest importance, and the great advantage 

 possessed by such a formula as that now under consideration is that relative dimensions 

 are made prominent. 



