— 9 — 



3. Number of branchiostegal rays. 



Owing to the small number of variants, the branchiostegal rays would at first 

 sight appear but poorly suited to the purpose of characterizing species. On the 

 other hand, they are peculiarly susceptible to the alizarine staining process, which 

 fact, together with the smallness of the number, renders it possible to rapidly and 

 easily deal with a large amount of material. For this reason, they have been 

 included in the present work. 



The investigations, which comprised the branchiostegal rays of both right and 

 left sides, (for results, vide Table IV, A and B) showed that these rays, despite the 

 smallness of their number (8—13), may exhibit considerable numerical variation. 

 True, the figure 11 is in all samples that of most frequent occurrence; the distri- 

 bution of the remaining variants, however, exhibits characteristic differences. Worthy 

 of note is also the fact that a difference of one ray may frequently be found 

 between the right and left sides of one and the same spectimen. There is no 

 definite rule as to which side in such cases has the greater number; this falls, 

 however, in most cases to the right, as is evidenced by the somewhat higher 

 average for this as compared with the left. In comparing samples from different 

 localities therefore, the rays of the right and left sides should be separately dealt 

 with, as has been done in the table, (IV, A — B). 



The table shows the averages for A. vulgaris (1 — 3), A. rostrata (4) and A. japo- 

 nica (5) as for the right side 10.777, 11.025 and 11.265 respectively, i. e., lowest for 

 the European, highest for the Japanese. The material in the case of A. japonica 

 is but small (83 specimens) whereas in the case of the other two species it is con- 

 siderable (1520 and 752). Despite the fact that the difference between the averages 

 is but slight, we yet find that there is a difference, as regards the number of 

 branchiostegal rays, betNveen the European, American, and Japanese eels. The 

 probable fluctuation of the average (P. F. A.) is here determined with peculiar 

 accuracy. We find, that the true average for A. vulgaris lies between 10.716 and 

 and 10.838, for A. rostrata between 10.934 and 11.116, so that the two fluctuations 

 do not overlap. It may be as well here to point out that this is equivalent to a 

 probability of 1400 to 1 in favour of the two species ditîering as regards the number 

 of branchiostegal rays. 



As already mentioned, 11 is the figure which most frequently occurs in the 

 case of all three species, whereas the figures next in order of frequency vary for 

 the different species. Thus in the case of A. vulgaris, 10 is far more frequent than 

 12; with A. rostrata, on the other hand, 12 is somewhat, and for A. japonica, 

 vevy considerably more frequent than 10. Even in the investigation of small 

 samples this feature is very soon apparent, the characteristic view of each species 

 being revealed at a very early stage of the work. A. vulgaris and A. japonica differ 

 considerably; to such a degree, indeed, that it is even easier to determine whether 

 a sample belongs to the one or to the other by examining the number of branchiost- 

 egal rays than by counting the vertebrae. On the other hand, the difference between 

 A. rostrata and A. japonica is less pronounceed. There is, however, approximately 

 the same difference between their averages as between those of A. rostrata and 



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