— 12 — 



another locality is that the seventh joint of first pair of legs 

 is a little shorter than in the second pair, while these joints 

 are of equal length in R. inermis. 



This detailed investigation proves that it is possible to 

 find in the development of first pair of legs every degree, 

 every connecting link, between normal specimens of 

 Rhoda inermis and specimens of Thysanoëssa neglecta 

 with the first pair of legs much elongate, strongly thi- 

 ckened and the number, length and thickness of setae very 

 altered ; furthermore that adult males of both « forms » 

 agree completely with each other in all features — except- 

 ing the anterior legs — though the terminal process of the 

 copulatory organs is very peculiar and their antennular 

 peduncles more modified, more richly furnished with 

 processes, spines and setae than in any other male of the 

 order. 



And the results are, that Rhoda inermis Kr. and Thy- 

 sanoëssa neglecta Kr. (T. borealis G. O. S.) are in reality 

 the same species, that the two genera must be united, that 

 Rhoda cannot be maintained, as no generic difference can 

 be pointed out between R. inermis and the single other 

 species, R. Raschii M. Sars, finally that according to the 

 rules of priority the name Rhoda must be dropped and 

 the species be named Thysanoëssa inermis Kr. 



It is, I think, without parallel among Arthropoda that 

 in the majority of the specimens of a species the legs are 

 normal, while in many specimens of both sexes, and at least 

 as much in young as in adults, one pair of legs are developed 

 more or less as a kind of prehensile organs, frequently even 

 much elongate and strongly thickened. The case may be 

 compared with the interesting mutations among the family 

 Atyidae first discovered by Bouvier and quite recently 

 excellently discussed by D r Caiman (i), but the differences 



(i) W. T. Calman : The Researches of Bouvier and Bordage on 

 Mutations in Crustacea of the family Atyidae (Quart. Journ. Micr. Sc. 

 Vol. 55, p. 785, 1910). 



