20 Georgina Sweet: 



diameter; all stages up to free embryos being found in the genital 

 tubes of the female larva, with very bluntly truncated head, and 

 gradually tapering to a very long fine point posteriorly, no sheath 

 being observed. (Larvae were insufficiently well preserved to allow 

 of accurate observation of histological structure, being also easily 

 broken in making smears.) 



In a previous paper on 0. gibsoni (Gilruth and Sweet, 1911) the* 

 very considerable amount of variation in important structures was. 

 pointed out, as had also been done by myself on two or three pre- 

 vious occasions (e.g, see Sweet, 1910, p. 243 et seq., and p. 24? 

 et seq.) when describing new species, which, so far as I am aware, 

 have not yet been recorded from elsewhere, and which are undoubt^ 

 edly and most naturally closely allied to analogous forms in the 

 same host in "older" countries. It would seem that the trans- 

 ference in these latter cases of the domesticated host from its older 

 habitat to a new environment in Australia has fairly quickly influ- 

 enced the structure of some of the contained parasites to an unex- 

 pected degree. On the other hand, although in most cases the older 

 workers gave no indication of any variability in measurements 

 given, it is probable that a considerable amount of variation exists 

 even, in well-known species elsewhere. In view of this it is of interest 

 while considering the value of measurements in specific diagnoses 

 to compare work summarised in a paper by Fracker (1914, p. 22), 

 in which he seeks " to ascertain the extent to which the proportions, 

 of the worm (O.ryurias vermicular is) were constant, and the parts 

 which undergo the greatest variation," considering especially the 

 use of the formula suggested by Cobb in 1890, and since invariably 

 used by him. Fracker concludes that while " an individual should 

 never be identified on the basis of the formula alone, or of the< 

 proportions alone," " the proportionate size of the organs in the 

 Nematoda is an important factor in their identification, and should 

 be stated in any description of a new species." With these con- 

 clusions, in so far as they emphasise the necessity for a statement 

 of measurements, most workers in the group of Nematodes will 

 agree, in view of the paucity of marked specific characters. 

 Curiously, however, in the case of the two species of Onchocerca* 

 which I have studied in this special connection, measurements pro- 

 portionate to the length of the body even of the male, such as are 

 emphasised by Fracker, are useless for the purpose of specific diag- 

 nosis, such measurements having no relation to one another, while 

 within a stated range, there is a marked similarity in certain 

 absolute measurements, quite irrespective of the length of the 



