SCIENCE-GOSSIP. 



255 



COLORATION AND VARIATION OF BRITISH 

 EXTRA-MARINE MOLLUSCA. 



By Arthur E. Boycott. 

 {Continued from page 228.) 



\ A " E may now pass on to the aquatic species P). 

 Here we find a few brightly. coloured forms, 

 Driissena, Anodonta, Unio, Neritina, Paludina and 

 certain forms of Sphaerium (Vs.'., 5. corneiim, var. 

 ..•.mm I and of Pisidium amnicum, with a large 

 majority with thin, horn-coloured, transparent 

 shells, without, as a rule and normally, any bands 

 or other markings: SegmeHtina, " Planorbis," Aplexa, 

 Pkysa. Amphipeplea, Limnaea, Ancylits, Vclhtia, 

 Bi'.hinu, Valvala, with Sphaerium and Pisidium, and 

 perhaps also the semi-amphibious genus, Succinea. 

 To consider these, it is necessary to glance at 

 the origin of the freshwater molluscan fauna (-). 

 It is a generally accepted fact that they have been 

 derived from marine forms by three possible 

 methods : (1) by direct migration up rivers : (2) by 

 inclusion of large or small areas of the sea, as salt- 

 water lagoons, which gradually become fresh ; (3) 

 by migration across the littoral, and subsequently 

 changing a terrestrial for an aquatic habit. In the 

 same way landsnails are derived from marine 

 forms by (1) taking to a terrestrial from a fresh- 

 water life ; (2) directly from the marine species. 

 It is a matter of very great interest, and also one 

 of considerable difficulty, to show the ancestors 

 and path of migration of our land and freshwater 

 mollusca, and this is not the place to enter into 

 the problem at any length. There are certain 

 points, however, which directly bear upon the 

 subject in hand, and must accordingly be men- 

 tioned very shortly. 



It is well known that many freshwater animals 

 have undergone a developmental modification ; 

 the details of the process vary, but the result is 

 that any small, weak, free-swimming larva is done 

 away with, and with it the danger of the species 

 being swept back again into the sea by 

 river currents. Some of the best-known examples 

 of this are : the parasitic glochidium la 

 A*uJ-nlJ and r 'm I t . the development of 

 in which the Zoom and Naupliui stages generally 

 found in marine Crustacea arc slurred over, ami 

 when the young r.rayfish first leavi 



■ 1 \ 11 



pp.79. 



»• L 



which is securely attached to and carried about by 

 the mother, it is sufficiently developed and strong 

 enough to cope with ordinary streams ; the 

 medusiform person in Hydra is reduced to a 

 mere gonadial excrescence on the hydriform 

 person ( 4 ). 



As a sort of corollary on these facts, \V. J. 

 Sollas ( 5 ) and K. Semper ( 6 ) have argued that' 

 no marine form could spread up a river from the 

 sea by means of its free-swimming larva, which in 

 the sea affords the chief means for its dissemina- 

 tion ; for the rough water and strong currents which 

 exist at the mouths of most rivers, especially where 

 a bar is well formed, would effectually prevent the 

 passage up the river of anything with such feeble 

 swimming powers as an ordinary larva. There is 

 doubtless a good deal in this objection, especially 

 when we take into account the strong inducements 

 there are for mollusca to pass up rivers (which will 

 be considered below), and the fact that the fresh- 

 water mollusca are, on the whole, an exceedingly 

 well-defined group. That it is not absolute is, 

 I think, shown by various facts which have been 

 observed in freshwater forms which live in 

 currents. There is no doubt that, while the 

 general rule holds good that freshwater forms 

 develop without a metamorphosis, there are not 

 infrequent exceptions to be noticed. Thus fresh- 

 water medusae are known, — the famous Limnocodium 

 sowerbii at Regent's Park, and Limnocnida from 

 Lake Tanganyika; and a third (llaimomises lacustris) 

 has been described by J. v. Kennel, from Trinidad, 

 living in a small freshwater lagoon, with a mixture 

 of Pnlychaeta and Mysil with frogs, Cliaetogaster, 

 Planorbis, Pliysa, and freshwater plants ("). Again, 

 among the Crustacea ("), Apus, Branchipus, 

 Esther ia, Cypris and Cyclops are freshwater species, 

 but have a free larva, while Cypridina, Nebalia, 

 and Diattylis, e.g., are marine, but have no 

 metamorphosis, The freshwater Polychaet, 

 Lumliricimtreis, has a free-swimming trochosphere. 

 I Korschelt (") has shown that the freshwater 

 tna has a free-swimming larva. It has been 



i'i A. M. Marshall Ii Biol, Lab., Oweni Coll., 



B. R. Lankeitei An. " 11 ydroi oa, 1 In 



Brit apt 11), p. 61, '1 if only fall i" m« ntli n 



thai Kli ind 1 M, Balfoui do n"i bold tbi imi 



c-i 1 > 



m " Animal Ufa " (1800), pp 1 '. , 



II p 



ch <•! itila Inl lurally l>< an d< 1 Ivi <l 



or' 1 ,1 1 - .[ n, ' 1 1 in '.'■ ,S,I islla 1 11 09), 

 ., , -u ■.' 1 ratlvo Embryol m 



I'i a. Nil 



