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Show 102 IIABITS OF WORMS. CHAP. II. passage by swallowing tho earth ; for it is incredible that the ground could yield on all sides to the pressure of the pharynx when pushed forwards within the worm's body. That worms swallow a larger quantity of earth for the sake of extracting any nutritious matter which it may contain than for making their burrows, appears to 1ne certain. But as this old belief has been doubted by so high an authority as Claparede, evidence in its favour must be given in some detail. There is no a priori improbability in such a belief, for besides other annelids, especially the Arenicola marina, which throws up such a profusion of castings on our tidal sands, and which it is believed thus subsists, there are animals belonging to . the most distinct classes, which do not burrow, but habitually swallow large quantities of sand; namely, tho Inol]uscan Onchidium and many Echinoderms.* If earth were swallowed only when worms deepened their burrows or made new ones, castings would be thrown up only occasionally; but in many places fresh castings may * I state this on tho authority of Semper, 'Reisen im Archipel der Philippioen," Th. ii. 1877, p. 30. Crru. II. EARTIT SWALLOWED AS FOOD. 103 be seen every morning, and the amount of earth ejected from the same burrow on successive days is large. Yet worms do not burrow ~o a great depth, except when the weather Is very dry or intensely cold. On my lawn the blac~ vege:ablo mould or humus is only about 5 Inches 1n thickness, and overlies lightcoloured or reddish clayey soil: now wlwn castin~s are thrown up in the greatest profuswn, only a small proportion are ligl1t ooloured, and it is incredible that the worms s~oul~ dai.l y make fresh burrows in every d1rectwn In the thin superficial layer of dark.-coloured mould, unless they obtained nutnment of some kind from it. I have observed a strictly analogous case in a field near my house where bright red clay lay close beneath the surface. .Again on one part of the Downs near Winchester the vegetable mould overlying the chalk was found to be only from 3 to 4 inches in thickness; and the many castings here ejected were as black as ink and did not effervesce with acids; so that the worms must have confined themselves to this thin superficial layer of mould, of which large quantities were daily swallowed. In |