322 



SCIENCE-GOSSIP. 



COLORATION AND VARIATION OF BRITISH 

 EXTRA-MARINE MOLLUSCA. 



By Arthur E. Boycott. 

 {Continued from page 292.) 



A MONG the freshwater Pelecypoda, the smaller 

 species (Sphaerium and Pisidium) have thin un- 

 ornamented shells, a fact which points to the same 

 conclusion here as in the Gastropoda, though some 

 small marine bivalves are quite plain. But all 

 the aquatic forms do not exhibit this monochro- 

 matic shell. There are five genera which are 

 more or less brightly coloured. The line of 

 argument just given is further supported by the 

 fact that four of these are very closely allied to 

 marine forms. Anodonta and Unto are not far from 

 several marine Pelecypoda. Dreissena (*) is an only 

 slightly modified Mytilus (this has been denied, 

 see ante p.' 291, Note 18), and is remarkable in still 

 possessing its free-swimming larva. Theodoxus 

 (Neritina) is very much the same thing as the 

 marine Nerita, and some species live in salt water. 

 These species then show their close alliance with 

 marine forms by their bright coloration. At the 

 same time it is only fair to mention two facts : 

 firstly, Anodonta is a very old freshwater genus, in 

 fact one of the earliest known, A. jukesii occurring 

 in abundance in the Old Red Sandstone at Kil- 

 larney, and no one seems to doubt the correctness 

 of the generic diagnosis. Secondly, the half-way 

 genus, Hydrobia, which lives in brackish water, has 

 a unicolorous, dull, unornamented shell, while it 

 is presumably migrating from the sea. The 

 remaining genus, Palndina, offers some difficulties. 

 Palaeontological evidence shows that they are a 

 fairly old (cretaceous) freshwater genus, and that 

 the highly-coloured forms have been derived from 

 much simpler ones since their immigration into 

 fresh water ('-). The immediate relationship to any 

 recent marine forms is by no means obvious. 

 On the other hand, ontogenetic evidence points to 

 a more elaborate ancestor— possibly the ultimate 

 marine one ; for the embryonal hairs with which 

 it is conspicuously decorated would indicate either 

 a hirsute or an ornamented line of descent. The 

 coloration, greenish and brown, may be cryptic 

 with green weeds and brownish mud, but this 

 hardly looks very likely. It is really rather curious 

 that Anodonta should be so often brightly coloured, 

 as it spends so much of its time deeply buried 

 in the mud. 



C. C. Fryer ( s ) has published some interesting 



(!) Only appears in Upper Eocene geologically. It retains 

 its bright colours even in the dark. Brit. Conch, i. p. 48. 



( ! ) A. R. Wallace: " Darwinism " (1889), p. 381. " Nature," 

 xiv. p. 275. 



( 3 ) " Conchologist," 1. (:Sgi), p. 25. 



notes on the colouring in our freshwater mollusca 

 He thinks that less competition explains the horn- 

 colour of Limnaeidae and the dull greens and 

 purples of Paludinidae and Unionidae, which, 

 however, he regards as "primitive." This (Darwin's) 

 is a probable view, for it is very difficult to show 

 that their ancestors had thick coloured shells, as 

 I think possible. He considers that these dull 

 greens and browns are protective, and aptly 

 compares the colours of freshwater insects. 

 Further, in Neritina fluviatilis the " chequered shell 

 and spotted hind part of the foot may aid it in 

 concealment among the fine pebbly sand ( 4 ) of swift 

 streams." Speaking of Limnaea peregra, he says : 

 " When the shell is thin and light-coloured, the 

 dark-bluish animal, variegated with yellow spots and 

 patches, is visible through the body whorl, and owing 

 to the irregular distribution of colours obscures the 

 outline and assimilates to the muddy bottom." 

 These are two very ingenious observations, but 

 they seem to be pushed quite as far as is advisable 

 in such cases. 



As far as I know, nothing further has yet been 

 suggested relative to the normal colours of our 

 aquatic species. None of them are transparent 

 enough to be protected in that way, which is so 

 common among marine forms, except perhaps a 

 few species when very young (Dreissena larva). As 

 a rule, even when quite small, they are pigmented 

 fairly strongly in one way or another. I once took 

 huge pains to extract a twelve-inch stone from a 

 very muddy pond under the impression that it was 

 the record Anodonta cygnea at last ; but I have not 

 heard of any similar instances. 



Certain caddis-cases (Helicopsyche) from Colorado 

 are said to very closely resemble Valvaia ( 5 ), but 

 how far this holds in England, or which way the 

 advantage, if any, lies, I do not know. 



We may next very briefly consider what is known 

 as to the meaning of the normal coloration of the 

 integument in the slugs ( c ). As mentioned above, 

 loss of an external shell is often followed by 

 protective colouring, according to Lang. Scharfi 

 considers that " the colours of slugs in Ireland 

 are at all ages, as a rule, protective." With this 

 Simroth agrees as far as the smaller species are 



(*) I have seen it mostly on large smooth stones (Hereford- 

 shire) and on water-weeds (Berkshire). 



p) " Young Naturalist." 1888, p. 17 : T. D. A. Cockerell. 



( 6 ) The following is largely a resume of the excellent work 

 of R. F. Scharfi, "The Slugs of Ireland," Sci. Trans. Roy. 

 Dublin Soc, (2) iv. pp. 513-562 (i8S8-i8g2). 



