The common liver-fluke, which in the winter of 1879–1880 cost Great Britain the lives of no less than three million sheep, is perhaps the best known of these remarkable parasitic forms of life. Its history shows us, in one important particular, how essential it is for the creature to meet, at certain stages of its existence, with the exact host to which it is accustomed. Unless the newly-hatched embryo finds a _Limnaea truncatula_ within about eight hours it becomes exhausted, sinks, and dies. It has been tried with all the other common pond and river Mollusca, with _Limnaea peregra_, _palustris_, _auricularia_, _stagnalis_, with _Planorbis marginatus_, _carinatus_, _vortex_, and _spirorbis_, with _Physa fontinalis_, _Bithynia tentaculata_, _Paludina vivipara_, as well as with _Succinea putris_, _Limax agrestis_ and _maximus_, _Arion ater_ and _hortensis_. Not one of them would it touch, except occasionally very young specimens of _L. peregra_, and in these its development was arrested at an early stage. But on touching a _L. truncatula_ the embryo seems to know at once that it has got what it wants, and sets to work immediately to bore its way into the tissues of its involuntary host, making by preference for the branchial chamber; those which enter the foot or other outlying parts of the _Limnaea_ proceed no farther.[143] Many similar cases occur, in which littoral Mollusca, such as _Littorina_ and _Buccinum_, form the intermediate host to a worm which eventually arrives within some sea-bird. Certain Nematode worms (_Rhabditis_) are known to inhabit the intestine of _Arion_, and the salivary glands of _Limax agrestis_. Diptera habitually lay their eggs within the eggs of _Helix_ and _Limax_. Many species of mite (_Acarina_) infest land Pulmonata. No adult _Limax maximus_ is without at least one specimen of _Philodromus_ (?) _limacum_, and the same, or an allied species, appears to occur on the larger of our _Helices_, retiring upon occasion into the pulmonary chamber. Several of the Crustacea live associated with certain molluscs. _Pinnotheres_ lives within the shell of _Pinna_, _Ostrea_, _Astarte_, _Pectunculus_, and others. Apparently the females alone reside within the shell of their host, while the males seize favourable opportunities to visit them there. A specimen of the great pearl-oyster (_Meleagrina margaritifera_) was recently observed which contained a male Pinnotheres encysted in nacre. It was suggested that he had intruded at an unfortunate time, when no female of his kind happened to be in, and that, having penetrated too far beneath the mantle in the ardour of his search, was made prisoner before he could escape.[144] _Ostracotheres Tridacnae_ lives in the branchiae of the great _Tridacna_. A little brachyurous crustacean inhabits the raft of _Ianthina_, and assumes the brilliant blue colour of the mollusc. =Means of Defence=