2 ' A Nematode Formula. 



necessity for exactitude and completeness in tlie matter of measurements, this 

 author, who has described a greater number of new forms than any of his 

 predecessors in the same field, and added also very essentially to our know- 

 ledge of the IS'ematode anatomy in general, has adopted a series of ratios by 

 which a considerable number of measurements are expressed very concisely. 

 These ratios he represents by the Greek letters a, /3, y, his a being the ratio 

 of the length to the median (greatest) diameter, /3 the ratio of the total 

 length to the length of the oesophagus and y the ratio of the total length to 

 the length of the tail. Thus, after having given the absolute length, he is 

 enabled to give three remaining dimensions by means of such an expression 

 as the following: a = 45, /3 = 5, y=8. 



The formula which I now propose points out, by means of eleven numbers, 

 eleven dimensions, and serves at the same time to indicate the sex from which 

 the measurements were taken, as well as the general form and size of the 

 sexual organs. The following are my formulae for a species of Onchol- 

 aimus : — 



Female. Male. 



1- 8-2 17-2 52 93-3^^^ "8 8' 16-6 M 94-1 ^,^^ 



•9 1-5 1-6 17 -8 -8 1-3 li 1-4 



The formula for the male is distinguished from that for the female by the 

 use of the letter M, for reasons presently to be stated. In order to exhibit 

 in as graphic manner as possible the nature of this new formula, I have 

 desired the printer to set up the above formula for the female in special 

 type as follows : — 



Pharynx. Nerve-ring. Base of Neck. Vulva. Anus. 



lengths !• 8-2 17-3 52 Q'S S i .77 m m 



Diameters '9 r5 re 17 .^^x^^. 



Pharynx. Nerve-ring. Base of Neck. Vulva. Anus. 



The numbers above the horizontal line relate to longitudinal measurements, 

 while those below it relate to diametral measurements. The first number 

 above the line (1') represents the distance from the anterior extremity of the 

 animal to the bottom of the pharynx or buccal cavity. The first number 

 below the line {'d) represents the length of the body-diameter which passes 

 through the base of the pharynx. The second number above the line (8'2) 

 represents the distance from the anterior extremity of the animal to the centre 

 oi the nerve-ring ; and the number directly below (1'5) represents the length 

 of the corresponding diameter, i.e.^ the body-diameter passing throtigh the 

 nerve-ring. The third pair of numbers (17"3 and 1'6) represent measure- 

 ments relating to the posterior end of the oesophagus or base of the neck. 

 In other wrirds, 17 3 is the distance from the anterior extremity of the 

 animal to the posterior end of the oesophagus, or is the length of the neck 

 including the head; and VQ is the diameter of the body at the point where 

 the oesophagus joins the intestine, i.e., where the neck joins the body. 

 Finally, 52 and 93"3 are the distances from the anterior extremity to the 

 vulva and the anus respectively, and 1"7 and '8 are the lengths of the 

 corresponding diameters. It will be seen that the different dimensions are 

 taken up in the formula in a natural order. Eeading the formula from left 

 to right reads off the dimensions of the animal from head to tail. Now 

 comes the peculiarity of the formula: The unit of measurement is not 

 ahsoJute but relative, is, in fact, nothing else than the hundredth part of 

 the length of the worm itself. In other words, the measurements are 



