SCIENCE-GOSSIP. 



115 



the shells of their companions ; in fact, Shuttleworth 

 goes so far as to say that he can recognize the 

 marks of the teeth of Neritina fluviatilis on other 

 shells, and Sowerby held that some acid was 

 developed from fermenting vegetable matter (i). 

 G. W. Shrubsole has shown that out of four waters, 

 examined three, which contained from 053 to 400 

 grains of lime per gallon, acted strongly on shells, 

 while the other one, which contained 833 grains, 

 had no action (2). But it does not appear whether 

 all the^ other constituents of these waters were 

 estimated, and four cases are not very many to 

 found an observation of this kind on. Something 

 of the same kind has been suggested by S. 

 P. Woodward {^), who says: "All fresh waters 

 are more or less saturated with carbonic-acid 

 gas. . . . But in the absence of lime to 

 neutralise the acid, the water acts on the shells." 

 This is of course no explanation of the loss of 

 periostracum. At first sight the electrolytic 

 process consequent on imperfect homogeneity of 

 the shell, suggested to J. G, Jeffreys (^) by W. 

 R. Grove, is an attractive idea ; but the action 

 should be stronger in the sea than in fresh water, 

 whereas, as a fact, erosion is much more common 

 among extra- marine forms. 



It seems rather hard to understand how the 

 "eggs of N. fluviatilis" C) or HgS could possibly 

 destroy any part of the shell's periostracum. It 

 seems to me that the initial cause of erosion (in most 

 cases at any rate) is not to be looked for in such 

 agencies ; I believe it to be a natural consequence 

 of a state which might be termed one of " general 

 ill-health," and the following case illustrates the 

 point. In April, 1893, a single large Limnaea peregra 

 was brought from some ponds close by and placed 

 in a small fountain -basin, about eight feet in 

 diameter by two feet or so deep. The descendants 

 of this specimen bred and flourished exceedingly for 

 some time. In the autumn of last year (1895), how- 

 ever, I noticed one specimen which was very slightly 

 eroded. In February, 1896, some dozen specimens, 

 apparently in good health and uneroded, were 

 placed in a small jar aquarium in the house, where 

 they seemed to do well at first. In April, I noticed 

 (they had not been under constant observation) 

 that some had died, while all that were left 

 were infected, to an extraordinary degree, with 

 Chaetogaster, and all were considerably eroded. On 

 examining the fountain, many dead shells appeared, 

 and nearly every specimen was more or less 

 eroded and infected with the parasitic woi-m. The 

 remainder of the specimens in the jar died by the 

 middle of May, after having been obviously ill. 



(') Gray's Turton, ed. 2 (1857), p. 46. 



(-) Journ. of Conch., v., p. 66 ; Camb. Nat. Hist., iii. (iSo-;) 

 p. 276. ■'•'" 



P) Manual (1851), p. 41. 



(*) Brit. Conch., i., pp. li.-liv. 



At the present time (August) there are very few 

 live peregra in the fountain, where before this 

 summer they were exceedingly abundant. 



This was the first batch of eroded peregra I have 

 come across, and the association of Chaetogaster 

 with them was very striking. 



How far Chaetogaster is injuriously parasitic on 

 snails I do not quite know. P. J. Van Beneden (•'-) 

 says it lives " at the expense of " mollusca ; F. E. 

 Beddard says that C. limnaei of Von Baer (= C. 

 diaphaniis. Oersted) "lives parasitically upon fresh- 

 water mollusca, and sometimes within their 

 bodies " (''). It does not, however, appear that, as 

 a rule, the worm actually feeds — at any rate 

 entirely — on the snails. I have frequently noticed 

 vegetable remains in their gut, and once a 

 specimen of Pediastrum (?). But at the same time 

 it would appear very probable that they are in 

 some way injurious to their hosts. It is possible, 

 of course, that the small volume of water in 

 which the peregra were placed acted injuriously, 

 but other specimens had been living more than a 

 year in a precisely similar aquarium without 

 showing obvious signs of ill-health. In another 

 similar jar some specimens of Planorbis coviplanatus 

 were placed, and, about the same time, developed 

 a pit erosion ('), and eventually most of them died. 

 Here again I think erosion was consequent on ill- 

 health, how caused I do not know. I failed to 

 find Chaetogaster, but, on the whole, Pla>torbis seems 

 to bear a small volume and bad conditions much 

 worse than Limnaea. 



I have somewhere seen it strenuously denied that 

 there is any "organic connection" between the 

 snail and the periostracum of its shell. But it 

 appears probable that such a connection, weak it is 

 true, may exist. When a snail dies, the periostracum 

 soon goes if it is exposed to the weather. If there 

 is no organic connection between the periostracum 

 and the snail, why should this be so ? And hence, 

 the connection may be partially suspended in ill- 

 health, as in the case of L. peregra above, and so 

 removal of the periostracum and consequent erosion 

 ensues. After the very severe frost early in 1895, I 

 noticed that deperiostracization was comparatively 

 common in living Tachea from an exposed railway 

 embankment. In Lamellibranchiata, the umbones 

 are nearly always — normally in fact — eroded. They 

 are the oldest part of the shell, and in that part of 

 the shell the connection with the living animal is 

 soonest given up and weakest. Once, so to speak, 

 the periostracum is dead, it is removed by natural 

 processes of decay, and the calcareous substance is 

 dissolved by the carbon dioxide in the water. 



As a last example I may adduce the following : 



(*) "Animal Parasites" (1S76) p. 114. 

 C') " Monograph of Oligochaeta " (1S95) p. 306. 

 (') A very similar case appears to be recorded in Journ, 

 of Conch, v., pp. 66-7 (G. W. Shrubsole). 



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