4 A Nematode Formula. 



One may quickly familiarize liimself with this new formula by imagining 

 that the horizontal line represents the animal under consideration, and that 

 the dimensions are written alongside, opposite the proper parts, — that is, 

 opposite the base of the pharynx, the nerve-ring, the base of the neck, the 

 vulva and the anus. 



Being already familiar with the use of this formula, I am not a proper 

 judge of the difficulties that would occur to a novice in its use. To me the 

 most ob^dous difficulty is that of having always to consider the anus with 

 reference to its distance from the anterior instead of, as is usual, from 

 the posterior extremity, a difficulty which should disappear with a little 

 practice. It is much easier to enumerate what seem to me the advantages of 

 the formula. 



(1.) The position of each number indicates the dimension to which it 

 refers. The formula is thus brief, yet concise. 



(2.) The position of the nerve-ring is indicated. In properly prepared 

 specimens the position of the nerve-ring is clearly to be seen, 

 so there is no longer any reason why the position of so important 

 an organ should not be entered among the characteristics. ]n order 

 to see at once the relative position occupied by the nerve-ring, 

 comparison should be made between the second and third numbers 

 above the line. It will be thus seen that in the species whose 

 formula has been given, the ring is situated a little in front of the 

 middle of the neck. 



(3.) A glance along the lower line of the formula reveals at once the 

 general form of the body. The worm whose formula has been 

 given has a somewhat cylindrical body. The portion of the neck 

 in front of the nerve-ring tapers considerably. The body also 

 tapers considerably in front of the anus. This latter is patent from 

 comparison of "8 with 52'1. The vulva is central, which means that 

 the sexual organs are probably double and symmetrical. Therefore, 

 the body would not be likely to diminish much in size in the 

 immediate vicinity of the vulva. Hence the decrease in size (1"7 

 to "8) must take place considerably behind the vulva, and therefore 

 near the anus. In very plump worms the largest of the numbers 

 below the line may rise as high 10, in slender ones become less than 

 unity. 



(4.) By averaging the specific formulae of a genus, we may obtain a 

 generic formula. During phylogenetic and systematic studies the 

 specific and generic formulae greatly facilitate the necessary 

 comparisons. 



The following description of a new species of Oncholaimus will illustrate 

 the use of the formula : — 



Oncholaimus index, n. sp. —. — f7 — 2^ — 2^3 — p-s-si. The thin cuticula is 

 destitute of striae. By careful use of high powers short submedian hairs 

 may be discovered throughout the length of the body. The conoid neck 

 terminates in a very slightly compressed head, which, at first glance, seems 

 to be entirely naked, but is seen after careful focussing to bear six very 

 short setae, two lateral and four submedian, all opposite the apex of the 

 longest pharyngial tooth. When the worm is seen in the dorso-ventral 

 aspect, the two circular lateral organs appear like two small oblique open 

 pockets opposite the middle of the pharynx. Six thin lips, each having the 

 form of an isosceles-spherical triangle, together form a flat-topped dome over 



