100 MERISTIC VARIATION. [part I. 



Passing now to the question of the distinctness of A. milhausenii, it seems clear 

 that, as Eathke said, it should never have been considered a distinct species. The 

 character of the finless tail, which is now seen to be one of degree, does not differ- 

 entiate it satisfactorily, and, as Schmankewitsch found, it is to be seen swimming 

 with tin-bearing individuals. It has never been shewn that there is a male A. mil- 

 hansenii, with distinctive sexual characters, and among the Branchiopoda the 

 various sexual characters of the second antennas in the male are most strikingly 

 distinctive of the several forms. While being in no sense desirous of disparaging 

 the value of Schmankewitsch's very interesting observation, I think it is misleading 

 to describe the change effected as a transformation of one species into another. 

 Schmankewitsch himself expressly said that he did not so consider it, and it is- 

 unfortunate that such a description has been applied to this case. 



The question of the division of the 8th post-abdominal segment of Artemia, 

 stated to occur on dilution of the water, directly concerns the subject of Meristic 

 Variation. As to the facts, there is no doubt that the tail of Branchipus appears to 

 be made up of seven segments besides the two which bear the external generative 

 organs, in all, nine, while in the commonest forms of A. salina there are only 

 eight such segments ; and that the difference lies in the fact that in the long 

 terminal segment of A. salina there is generally no appearance of division. But as 

 Claus 1 has shewn, the last apparent division in Branchipus is of a different 

 character from that of the other abdominal segments. This is indeed easily seen 

 in B. ferox, B. stagnalis, B. spinosus, &c, in which the appearance of the last 

 division is very different from that of the other divisions. It appears, in fact, to be 

 rather an annulation than a segmentation. In longitudinal sections the distinction 

 is quite clear. Such a division, according to Schmankewitsch, appears in the third 

 generation of A. salina bred in diluted salt water. 



Among my own specimens an appearance of division in the last segment occurs 

 in a considerable number, and these are not by any means from the most dilute 

 waters alone, some of them being from waters of great concentration. For instance, 

 the specimens in XXIX, LI, XXXVII, XXXIX and XIV, all have no trace of such 

 division. On the other hand, it was found in several specimens from XXVI (Sp. G. 

 1*179) and XLIII (Sp. G. 1'115), while others from these localities did not shew it. 

 These facts relate to adult females bearing eggs. I do not think, therefore, that the 

 relation of this appearance of division to the salinity of the water is a constant one. 



Lastly, as regards the relation of Artemia to Branchipus, Schmankewitsch has 

 maintained that the division of the last abdominal segment is the only structural 

 character really differentiating Branchipus. Claus (I. c.) pointed out that there are 

 many other points of difference, and that the supposed division is not a structural 

 character of great moment. But above all these, it should be remembered that by 

 the sexual characters of the males, Branchipus is absolutely separated from Artemia. 

 No Branchipus has any structure at all resembling the great leaf-like second antennae 

 of the male A. salina or A. gracilis- Verrill. Schmankewitsch remarks (Z. f. iv. Z.,. 

 1877, p. 492) that there are species of Branchipus (e.g. B. ferox) without the 

 appendages characterizing the second antennas of B. stagnalis s , &c, and that the 

 males of Artemia bear on the second antennas a knob, which is possibly the repre- 

 sentative of the appendages of Branchipus, but nevertheless there is no resemblance 

 whatever between the males of B. ferox or of any other Branchipus and those of 

 Artemia, and there is no reason to suppose that these sexual characters are modified 

 by the degree of concentration of the water. The statement that the descendants 

 of an Artemia can be made to assume the characters of Branchipus Schaffer, depends 

 entirely on the acceptance of Schmankewitsch's criterion of that genus, which is set 

 iip in practical disregard of the far more distinctive sexual characters. It is, besides, 

 as has already been stated, only an irregular and possibly misleading relation which 

 subsists between this appearance of segmentation and the salinity of the water 3 . 



1 Anz. Ak. Wiss. Wien, 1886, p. 43 ; see also idem, Abhandl. Gdttingen, 1873, 

 Taf. in. Fig. 10, Taf. v. Fig. 16. 



2 For two samples of this American form I am indebted to Dr A. M. Norman, 

 who received them from Professor Packard. 



3 I cannot leave this subject without expressing astonishment at the com- 

 paratively slight and evasive differences in the structure of Artemice and other 

 Crustacea inhabiting waters of different salinity and composition. It is not a little 



