I902. Kane. — Irish Natural History. 57 



kochia7tus, in the wells about Dublin. Country members con- 

 gratulate their city friends, and trust it may not be exter- 

 minated. Dr. Scharff has also taken an Isopod new to the 

 British Islands, Arniadillidium pulchellum, and by further 

 researches has satisfied himself that the land planarian worm 

 Rhynchodemus scharffii may now be confidently reckoned an 

 indigenous native of Ireland. 



And now, perhaps, I may ask your patience when I refer to 

 one or two results of my own studies in regard to our freshwater 

 Crustacea. In 1861 Professor Ivoven startled the scientific 

 world by announcing that in the two great Swedish lakes, 

 Wetter and Wener, he had taken five species of Crustacea 

 which were to all intents and purposes identical with marine 

 species. One of these was a Mysis — a freshwater ** shrimp." 

 The Mysis is an interesting genus, for zoologists know that in 

 the life-history of the Crustacea, the immature animals go 

 through a series of transformation stages after emerging from 

 the ^%%^ and one of these is called the Mysis stage. That is 

 to say, that the young Penaeus, for instance, in its transforma- 

 tions develops into forms which have adult representatives, 

 the last of which is the Mysis, and then finally changes to 

 the Penseus. The genus Mysis does not undergo any further 

 change, but passes its adult life at that stage. From this and 

 numerous other similar phenomena a dictum of the theory of 

 evolution has arisen j namel}^, that the life-history of the 

 individual recapitulates the history of the development of the 

 race. Hence we infer that the Mysis, the lyobster, the Penaeus, 

 and so on, are highly developed forms that originated from a 

 common and simpler organised ancestor. 



Mysis oculata is a marine species which frequents the Arctic 

 seas from Spitzbergen to I^abrador. It is scarcely distinguish- 

 able from Professor Loven's freshwater Mysis relicta. And 

 since in these two Swedish lakes there are four other fresh- 

 water Crustacea also closely similar to sea-dwelling species, it 

 has been thought that probably all five were introduced into 

 these great inland basins by a submergence of land during 

 the Glacial period, and were captured as it were by the upper 

 movement which followed subsequently. 



But latterly some have hesitated to accept this hypothesis^ 

 not only geologists who dispute the submersion of certain 



