200 Miscellaneous. 



whicli was mentioned above) an old arm of the sea which has been 

 separated from the Gulf of Finland and whose waters have 

 gradually lost their saltness. According to von Kenuel the 

 !N^emertean found by him in the Embach is very closely allied to 

 Tetrastemma obscurum of Max Schultze, a species which is freely 

 marine in the North 8ea, but which, on the other hand, is found 

 to be the only one capable of enduring the extreme reduction in 

 saltness of the waters of the Gulf of Finland. This species has been 

 encountered as far as Revel and Helsingfors. It, lives in tliis region, 

 in a medium which is scarcely brackish, in company with Planarians, 

 UligochaBtcs, and various distinctly fluviatile types *. May we not 

 fairly conclude from this that, if not T. ohscuriun, at least one or 

 more allied forms have become little by little and definitely accus- 

 tomed to fresh water, and have become distributed there in time 

 and by degrees, as is the rule in the case of fluviatile animals ? 



I would add that Nemorteans appear to enjoy quite a special 

 plasticity for adapting themselves to the most varied conditions of 

 existence. At the present moment four terrestrial species are 

 known. The first of these (Geonemerfes ixdaensis) was reported in 

 1863 from the Palaos Islands, Micronesia, by Prof. C. Semper. 

 Ten years later Willemoes-8uhm discovered a second (Tetrastemma 

 agrkola) at the Bermudas during the ' Challenger ' expedition. In 

 1879 G. Gulliver described Tetrastemma rodericianum, which he had 

 fojnd in the Island of Rodriguez (Indian Ocean), and almost simul- 

 taneously Prof, von Graff published an excellent study upon Geo- 

 nemertes diaJicophora f. The patria of this latter species is still 

 unknown ; like several Oligochaetes or terrestrial Planarians, and 

 the famous freshwater Medusa which was discovered in Loudon in 

 a tank in Regent's Park %, it was taken alive in the palm-house of 

 the Botanical Gardens at Frankfort on the Main at the foot of a 

 Cory2>ha, which had come without doubt from Australia — a fresh 

 proof of the facility with which organisms which are apparently the 

 most delicate are capable of disseminating themselves. 



These facts speak for themselves : they enable us to follow and 

 to understand the process, slow no doubt, but continuous, by which 

 first the fresh water and then the dry land have become peopled in 

 the course of ages. Their force increases owing to the very fact of 

 their being grouped together, and I therefore think that a note like 

 this, though it should diminish the surprise caused by certain disco- 

 veries, is not entirely destitute of interest from a general point of 

 view. — Comjjtes Rendus hehdomadaires des seances de la Societe 

 de Biologic (Seance du 30 avril, 1892) : from a separate impres- 

 sion communicated by the Author. 



* Max Brauu, Arch. f. Naturkunde Liv-, Esthl- u. Kurlands, 2 Folge, 

 Bd. X. (1884) ; Axel Spoof, ' Turbellaria, Discophora, et Oligochseta 

 fenuica,' 1889. 



t Zeitschr. f. wiss. Zool. Bd. xiii. (1863) ; Ann. & Mag. Nat. Hist. 

 ser. 4, vol. xiii. (1874); Phil. Trans, vol. clxviii. (1879) ; Morphol. Jahrb. 

 Bd. V. (1879 j. 



X Bipalixmx keivense, Moseley, of the hothouses at Kew Gardens &c. 

 Vide J. de Guerne, ' Excursions zoologiques dans les lies de Fayal et Sau 

 Miguel (Azores),' Paris, 1888, Chap. ix. See likewise J. de Guerue, 

 ' Medusas d'eau douce et d'eau saumatre ' &c., Bull, scient. dep. du Nord, 

 vol. xii. (1880). 



