the point appear to be required, but I can hardly think that anything other than negative results will be obtained.' Four warty-newts, placed in a vessel of water by Mr. C. Robson, extruded a number of Pisidia, the shells of most of which were open, with the animal extracted; four, however, were closed, and three of these contained young; the adult animals had been extracted even from these closed shells, but, in some of the young, the animals were still within the shells, yet it does not appear that they were actually observed to be alive. As far as diffusion by animals is concerned, it seems clear, I think, that the creatures are chiefly carried while very young, adhering to the feet and feathers of aquatic birds, or attached to plants thus adhering. Adult and partly grown shells, however, are liable to occasional transportal by creatures of various kinds, and, through the agency of birds, adult bivalves, even some of the larger kinds, may sometimes be carried through considerable spaces; but before giving the facts on this head we will consider the possible dispersal of fry and ova.

Mr. Darwin has a statement at page 344 of the Origin" that the eggs are not likely to be transported by birds; but Mr. Wallace, Professor Tate, Lieut.-Col. Godwin-Austen, and other writers have referred to

The eggs of Limnæa auricularia, it has recently been stated, have passed unharmed through the digestive system of swans. See "Zoological Record," xxviii. (1891), Moll. p. 37, referring to Pascal, "Journ. de Conch.," xxxi. pp. 9-15.

2 C. Robson," Science Gossip" for 1875, p. 220.

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such transportal as probable,' and Canon Tristram, in the winter of 1856-7, it is interesting to find, had the good fortune to discover the eggs of some molluscprobably Succinea-attached by their glutinous coating to one of the feet of a passing mallard shot by him in the Sahara, a hundred miles from water; thus, he remarks, such a bird "might easily carry a Succinea or Physa from Europe to the lakes of Central Africa." It ought to be remembered, however, that eggs of some kinds, thus exposed to the atmosphere until dry, would become very firmly attached and so remain even when again moistened, so that they would not be easily dislodged in a new locality; but, as many birds probably travel during gales at the rate of thirty-five miles an hour or much faster, eggs might certainly be carried before drying to considerable distances, and then when again dipped in water would soon become detached. Many kinds attach their eggs to aquatic plants, fragments of which, we shall see, are likely to be carried by birds. Something may possibly have been done by insects, for Mr. Standen informs me that he once saw, in the Hollinwood canal, an egg-capsule of the river-limpet (Ancylus fluviatilis) attached to one of the wing-cases of an Acilius, a strong flying water-beetle. Some eggs, it

"Island Life," 1880, p. 76; ed. 2, p. 79; R. Tate, "Land and Fresh-water Mollusks," 1866, p. 188; H. H. Godwin-Austen, "Field," lxvi. (1885), 499.

2 See "Zoologist," (3), i. (1877), 260-1.

3 This happened, at least, to eggs of Limnæa auricularia, which I exposed on a tin tray and on fragments of water-weeds.

"Origin," p. 326.

should be noted, from the localities in which they are deposited or the nature of the objects to which they are usually attached, are not at all likely to be dispersed by animals, and it will be remembered that some molluscs are viviparous. The fry, probably, are transported more frequently than ova. Those Unionida whose larvæ are parasitic for a time upon fishes are eminently liable to wide dispersal, at least, through waters in any way communicating with their habitats. The larvæ of Anodonta swim by the flapping of the valves of their shells, trailing at the same time a long byssal filament, and weaving themselves together in masses which spread out like a net. Little fishes, stirring up the masses, carry many larvæ away, the creatures managing to effect a hold-chiefly to the fins-by means of the toothed edges of their shells. They then become encysted, and are nourished, it is stated, by the exudations of the fish. Ultimately, on the splitting of the cyst, they drop out and fall to the bottom. In aquaria, Anodonta which have retained their brood in long quiescence, have been observed to let it out immediately on the introduction of fishes. The larvæ of Unio, differing in certain respects, are said to be inhaled by fishes, and are usually found attached to the gills. Whether these fry are likely to be carried

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1 E. Ray Lankester, art. Mollusca, "Encyclopædia Britannica," ed. 9, xvi. (1883), 694; and see also Dr. Schierholtz's memoir as to the development of the Unionida, Imperial Academy of Science, Vienna, 1888, my knowledge of which is derived solely from a leaflet printed by Mr. Standen, in explanation of specimens illustrating the life history of Anodonta, shown by him at one of the Manchester Microscopical Society's soirees.

from one piece of water to another by birds, etc., I am unable to say, for I have not seen it stated that they are able to withstand exposure to the atmosphere. The same remark applies to the free-roving pelagic larvæ of Dreissena. The just-hatched young of certain univalves, we know, are well suited for such transportal, for they cling firmly, and can live out of water for hours. On this head we have Mr. Darwin's celebrated and often quoted experiment with a duck's feet in an aquarium :

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"I suspended the feet of a duck in an aquarium, where many ova of fresh-water shells were hatching ; and I found that numbers of the extremely minute and just-hatched shells crawled on the feet, and clung to them so firmly that when taken out of the water they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off. These just-hatched molluscs, though aquatic in their nature, survived on the duck's feet, in damp air, from twelve to twenty hours; and in this length of time a duck or heron might fly at least six or seven hundred miles and if blown across the sea to an oceanic island, or to any other distant point, would be sure to alight on a pool or rivulet.2"

Unfortunately, however, I cannot find, after inquiring of a number of experienced ornithologists and sportsmen, that the creatures have ever been seen adhering to birds shot on the wing; Mr. Dresser, for instance, though he has shot thousands of aquatic birds in

' Univalves, as Mr. F. Darwin informs me.

2 66 Origin," p. 345.

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various countries, has never seen shells attached, and Mr. O. V. Aplin, the Rev. J. C. Atkinson, Mr. Abel Chapman, Mr. Cordeaux, the Rev. H. A. Macpherson, Mr. T. H. Nelson, and Mr. Howard Saunders have also replied in the negative to my inquiries on the point. But from general considerations, and especially in view of Mr. Darwin's experiment, it can hardly be doubted but that very young molluscs do thus adhere somewhat frequently, and perhaps the absence of observations made in the field is not surprising, for, as Mr. Layard remarks to me, the creatures are hardly likely to be noticed unless specially looked for, and this I suppose is very rarely done; indeed Mr. Aplin, replying that he had never seen young molluscs on the feet of water-birds, carefully added that he had never looked for them. Mr. F. Norgate, it is interesting to find, once noticed a small grey leech clinging to the plumage of a mallard shot by him. Mr. Macpherson remarks that widgeon, etc., sitting all day long half awake in the wet sedge, are very likely to carry shells on their feet or feathers when they fly at night to their feeding-grounds, and as Dr. Kobelt has suggested, the creatures will have a good chance of adhering also to the feet and legs of herons and other birds which often stand motionless in water for hours. Even adult shells of the smaller kinds may often attach themselves, but these, I suppose, in most cases, are soon shaken off. Mammalia frequenting fresh water, otters for instance, seem likely to 1 1 W. Kobelt, "Fauna der Nassauischen Mollusken," 1871,

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