256 



SCIENCE-GOSSIP. 



ascertained that the adults here possess some 

 small powers of locomotion. Reichel ( 10 ) states 

 that in winter they retire into deeper water, and 

 Korschelt has seen them climb up the sides of an 

 aquarium by means of their foot, and very slowly. 

 But its main mode of dissemination over long 

 distances, at any rate where it cannot be carried 

 about on convenient timber and barges, is no 

 doubt by means of this larva ; and it spreads and 

 lives in currents of very considerable rapidity ( u ) : 

 thus in the watermains of Paris nine species (in- 

 cluding three hitherto unknown ones) have been 

 found ( 12 ) ; and they have been seen in large 

 numbers in a similar situation in London ( I3 ). 

 These instances tend to show that an apparently 

 very weak larva may escape being swept away 

 by strong currents, and is capable of progressing 

 against them. Some small freshwater animals 

 (e.g., Cyclops) can resist the current of a suction- 

 tube in a much more efficient way than might be 

 supposed. 



The animals given above do not all live habitu- 

 ally, or even as a rule, in streams, many regularly 

 inhabiting ponds and quiet waters. It is very 

 possible that some of them have been derived 

 from marine ancestors by the conversion of 

 shallow, continental seas into freshwater lakes, 

 and by the inclusion of small or large areas of 

 salt water as freshwater lagoons. W. J. Sollas 

 is of opinion that this is the means by which the 

 freshwater fauna has been derived: and this is 

 very probably the case, though the probability of 

 some direct migration up rivers must not be 

 excluded. 



It is very interesting to note certain examples 

 where these processes seem to be going on at the 

 present day. In the Baltic, between Drago and 

 Papenwick, Mytilus edulis, Cardium edule, Teilina 

 balthica, Mya armaria, Littoritia rudis and Hydrobia 

 balthica live with Unio, Sphaerium, Neritina, Limnaea 

 and Bithinia (Braun.). At Stockholm Cardium, 

 Teilina, Limnaea peregra and Physa fontinalis live 

 together (Lindstrom). Near Gothland Limnaea 

 is found in the open sea at eight to twelve fathoms, 

 with Cardium and Teilina, and in the Frische Haff 

 Mya armaria lives alone of marine species, with 

 Limnaea (6), Physa (1), Planorbis (9), Ancylus (1) 



(M)Zool. Anz. x. (1887), p. 481. J. Frenzel (Biol. Centralbe, 

 xvii. (1897), p. 147; Journ. Roy. Micr. Soc, April, 1897, p. 

 117) maintains that colonies can move about en bloc without 

 separation of individuals ; they thus move into deeper water 

 in cold weather. Movement is effected by the younger 

 members of the colony (in which the foot is not yet 

 antrophied), by (1) fixing the foot and drawing their body 

 after it (as in Anodonta); (2) by pushing with their foot 

 behind ; and (3) by napping their valves. 



C')C/., H. W Kew, "Dispersal" (1893I, p. 217. 



( l2 ) " Revue Scientitique " (3I, xxvi. (1893), pp. 531-534: a 

 review of M. A. Locard's " Malacologie des Conduites d'eau 

 de la Ville de Paris." Conchologists who have had any 

 experience of modern French workers, and especially M. 

 Locard, will understand how to take the large number of new 

 and separate " species." 



(i 3 )Brit. Conch., i.,p. 48. 



Valvata (4), and Sphaerium (Mendthal), 2 ("). 

 It is well known that Net itina fluviatilis lives in 

 quite salt water with Mya armaria, e.g., in Loch 

 Stennis, Orkney p). The Baltic, where all the 

 marine and freshwater forms mentioned flourish 

 together, is still connected with the sea ; but there 

 are several examples known where inland bodies of 

 water show an admixture of marine and freshwater 

 mollusca. The Caspian is one of them. There 

 we find Neritina, Bithinia and Planorbis, with 

 Micromelania, Caspia, Clessinia, Ntmaturella, all of 

 which are modified forms of the marine Rissoidae ; 

 and among the Lamellibranchiata there are A nodonta 

 and Dreissena with Adacna, Didacna, and Monodacna, 

 which are derived from Cardium edule, which also 

 occurs. In Lake Tanganyika ( 16 ) several of the fresh- 

 water genera have a distinctly marine facies, such as 

 Tithobia, Neothauma, Limnotrochus (which possesses 

 all the outward aspect of the marine genera 

 Trochus and Echinella), Synolopsis (like Obeliscus 

 or Syrnola) ; Planorbis, Paludina, Physa, Unio, etc., 

 also occur. The elevation of Lake Tanganyika 

 is now 2,700 feet above sea level ( 17 ), and its 

 connection with the sea must have been in very 

 remote time. It is curious that the fauna of 

 the Victoria Nyanza (altitude, 3,900 feet), which 

 belongs to the same lake system, "appears to be 

 quite Nilotic, and no such remarkable forms as 

 occur in Lake Tanganyika have as yet been met 

 with." ( 18 ) Indeed, Pelseneer goes so far as to 

 deny any marine appearance in the Tanganyika 

 fauna. ( 10 ) Quite recently a special expedition has 

 visited these lakes to examine the fauna. I have 

 not yet seen the full results, but according to short 

 notices which have appeared in several journals ( 20 ), 

 J. E. S. Moore has determined that some of the 

 Crustacea and other groups are of a distinctly 

 marine type. 



Cases illustrating the transition from aquatic to 

 terrestrial forms are also known. Thus W. A. 

 Herdman has shown that Littorina rudis actually 

 lives better in air than water ( 21 ), and in Jamaica 

 A. H. Cooke has found several species of the same 

 genus living away from immediate contact with the 

 sea, in trees, etc. Littorina is also very resistant to 

 desiccation ( 22 ). Neritina, too, lives sometimes on 

 land as well as in fresh or salt water. What, 

 exactly, the curious pulmonate Onchidium is doing 

 seems doubtful. Limnaea offers at least one example 

 where we have freshwater amphibious mollusca, 



( H ) I take these examples from A. H. Cooke, "Concho 

 logist," ii. (1893), p. 42. 



( Is ) J. McMurtrie, quoted by R. Rimmer, op. cit. p. 24. 



( IC ) See E. A. Smith in Proc. Zool. Soc, 18S0, p. 344, and 

 :88i, pp. 276 and 558. 



('") C. Reid, " Natural Science," i. (1890), pp. 117 foil. 



(">) E. A. Smith, A. N. H. (6), x. (1892), p. 152, Proc. Zool. 

 Soc. (1877), p. 712. 



( 19 J See "American Naturalist," xxi. (1887), p. 2S8. 



( 20 ) e.g. Nature, lvi. (1897), p. 198. 



( 21 ) Proc. Liverpot 1 Biol. Soc, iv. (1890), p. 50. 

 ( 3S ) Camb. Nat. Hist., iii. p. 20. 



