138 . NEMATHELMINTHES chap. 



muscles, the character of the tail in the male, especially w lu-i 

 papillae are present, the number and the size of the spicules, and 

 the arrangement of the lips and mouth-parts generally. 



Cobb l has recently devised an ingenious formula in which 

 measurements of different parts of the body appear as percentage? 

 of the whole length of the body. The nature of this will b 

 understood by reference to Fig. 68. Such a formula should, 

 however, be used with caution, since it rests on the assumption 



Fig. 68. Diagram to explain the descriptive formula used for Nematodes. (From 

 Cobb.) 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 U 2 3 50 88 



C 7 8 10 6 



The unit of measurement is the one-hundredth part of the length of the worm. 

 The measurements are therefore percentages of the length. 



The measurements are taken with the animal viewed in profile ; the first is taken 

 at the base of the oesophagus, the second at the nerve-ring, the third at the cardiac 

 constriction, the fourth at the vulva in females and at the middle in males, the 

 fifth at the anus. 



that the proportions of the various parts of the body are constant 

 in different individuals, and it is by no means certain that this 

 is the case. 



Taking everything into consideration, it has seemed advisable 

 in the following systematic account of the Nematoda to abandon 

 the larger groups, and to deal directly with the families. Glaus 

 distinguishes seven of these, and the diagnoses given at the head 

 of each are mainly taken from his Grunclzuge der Zoologie. 2 



I. Family Ascaridae. 



Body rather stout. A dorsal and two ventro-lateral lips, 

 bearing papillae. Buccal cavity, distinct, seldom provided with 

 chitinous armature. The oesophagus often has two dilatations. 



1 Macleay Memorial Volume, Sydney, 1893, p. 252 ; and Proc. Linn. Soc. N.S. W. 

 2nd ser. vol. v. 1890, p. 449. 



2 4th edition, 18S0. 



