SCIENCE-GOSSIP. 



^57 



while some members of the genus (Limnata stagnate) 

 are quite aquatic. L. pa'.ustris will often crawl 

 out of an aquarium and live for some time w ithout 

 any moisture, and L. truncatula is really more a 

 terrestrial than an aquatic species. As far as my 

 experience goes the latter species is much more 

 often found sitting just outside the water on the 

 mud than submerged. It also possesses wonderful 

 powers of resisting drying up, which, as it is 

 frequently found in the merest trickle of water, 

 must often be of use to it P). It looks very much 

 as if L. truncatula were migrating one way or the 

 other : either leading allied forms out of the water, 

 or following them from land to an aquatic habitat. 



It is quite an open question whether the land 

 mollusca are immediately derived from fresh- 

 water or marine species. Littorina and Neritina 

 give some weight to the view- that some, at any 

 rate, terrestrial forms have had their immediate 

 ancestors in the sea ; while Limnata might be held 

 to point the other way. It seems a not uncommon 

 opinion that, while the terrestrial operculate forms 

 may very likely have come from the sea, the 

 Helicoidea are of freshwater origin. T. H. Mont- 

 gomery (-*) makes the general statement that 

 •' land forms are derivatives of freshwater forms in 

 all those groups of the Invertebrates which are of 

 marine origin, e^., the land pulmonate mollusca." 

 Perhaps these views are upheld by the fact that 

 while the lung in Cyclostoma and Limnaeidae is a 

 modified gill-cavity and retains a trace of the 

 lung-stuff in the adult, in the Helicoidea there is 

 no such trace, and it has been held (Von ihring; 

 not Semper) that the lung-cavity here is not 

 homologous with that of Limnata, but is a 

 dilatation of the duct leading from the excretory 

 pore to the exterior|-). On the other hand, it is 

 difficult to see how Limnata came to abandon a 

 branchial for a pulmonate mode of breathing if it 

 has never passed through a terrestrial stage, which 

 would have compelled it to do so. W. J. Sollas( a; ) 

 says : " The Helicidae are amongst the nearest 

 allies, and are probably ancestors of the Lim- 

 naeidae." The genital apparatus of Amphibulima 

 {Suuiiua) much resembles Limnata, and " the 

 radula of Limnata also supports the idea that it 

 is derive] from some Helicid form" (E. W. W. 

 Bowell) 



What is the reason why any marine forms have 

 left the sea and taken to a tcrreMri.il or a Bnviatile 

 life - It i-. undoubtedly due to the keen (lm| 

 existence which goes on in the densely-populated 

 shore water*. There itan extraordinary abundance 

 of life in the shallow waters of the sea. There 



1 



11 10. 



are in all probability numerous examples of pro- 

 tective and aggressive modes of coloration to be 

 discovered, though not very many have up to the 

 present been made out. It is plain, for instance, 

 how the general transparency of larval forms must 

 assist them. One of the most interesting cases of 

 this is perhaps the absence of haemoglobin in 

 Leptoceplialus, the transparent immature marine 

 form of the common eel. The mollusca no doubt 

 are also engaged in this warfare. They suffer from 

 the attacks of almost innumerable enemies, which 

 will swallow any snail they may come across. 

 J. G. Jeffreys ( a7 ) gives a considerable list, includ- 

 ing sea-birds ('•*), fish, Crustacea, star-fishes, and 

 even other mollusca (e.g., Buccinum, Purpura), and 

 mentions that the number of the small bivalve 

 Turtonia minuta taken from the stomach of a mullet 

 from Lough Larne was estimated by Hyndman 

 at 35,000. Even terrestrial mammalia have been 

 known to destroy mollusca. Rats are very fond 

 of Patella, and will eat large numbers (-'). Big 

 fish, such as the cod, eat large numbers, and it is 

 said that their stomachs are an excellent hunting- 

 ground for deep-water species, while the walrus 

 feeds almost entirely on Mya. 



Now the majority of marine species are more or 

 less highly coloured or ornamented ; it may be in 

 consequence of the strenuous struggle for existence 

 which is constantly going on. On the other hand, 

 only a certain number of cases of coloration have 

 been assigned to the usual causes — epigamic, 

 sematic, or cryptic (*>). A. H. Cooke has pointed 

 out that Strombtis mauritianus and S. luhuanus closely 

 resemble the genus Conns in general appearance. 

 Conus is a carnivorous genus with large teeth with 

 which they can inflict severe and poisonous wounds, 

 while Strombus has weak teeth and is as inoffensive 

 as mollusca usually are. It is hence probably 

 protected by mimicking the more offensive genus, 

 Conus ( ra ). Examples of protective colouring are 

 found in several instances. H. L. Osborn (• la ) 

 describes how Ovuluni uniplicatum varies between 

 yellow and red (shell and mantle), according as it 

 lives among yellow or red Leptogorgia. He is of 

 opinion that there is really vastly less mimicking in 

 the sea than on land. Certain points in this matter 

 of shell-colours have been carefully studied in some 

 marine genera (Voluta, Strombus, Conns), by Maria 



(i) •• British Concbology," i. p. lix. 



■• A. i>iiiii(..ici stirlbutei • l ir- great diminution In 



numbers of cocklci and mussels in Morecambi Miyiothr 



in the ss S'blrds consequent on protection by Act ot 



Parliament, lourn. of Conch, ifl, p, 76. 



{*•> A. H. Cooke, op. hi., |.. ■// and pp, 36 ;-.| generally, on 



nils* co cases where the colouring Is In the 



1st Interi Ulna oasei "i protective 



, have bean pointed out among the Nudlbranchlata, 



-■i.iiiy b) •'■ Gai 'ir. I Marine. Biol, Assoc., 



ind "". hologiit," II. (1893), p u 



W.A. Herdmsn, Q ( 1 , xx»l. <>Boo), p. 41, and Proc. 



Iv. I lH*j-,), 1 

 ■ p ., 



Pro* ' 1 Boc, vlil. p ir. 



K 4 



