102 HABITS OF WORMS. Chap. IL 



passage by swallowing the eartli ; 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 me 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 the mollus- 

 can Oncliidium and many Echinoderms.* 



If earth w^ere swallowed only when w^orms 

 deepened their burrows or made new ones, 

 castings would be thrown up only occasion- 

 ally ; but in many places fresh castings may 



* I state tins on the autliority of Semper, ' Eeisen iin 

 Archipel der Philippinen," Th. ii. 1877, p. 30. 



