192 THE BIOLOGY OF THE SEA-SHORE 



Among segmented worms, the large and enormously 

 abundant lug-worm {Arenicola marina) of mud-flats and 

 cockle-beds burrows to a depth of two feet, swallowing 

 sand as it goes. Their number at Holy Island has been 

 estimated at over 82,000 per acre, and the amount of earth 

 brought to the surface in their castings every year, at 191 1 

 tons per acre (Davison, 1891). Probably the greater part of 

 the contained organic matter is removed from this amount of 

 sand in its passage through the gut of the worms and the 

 sand is further purified by the action of water in the burrows. 

 The worms thus play an important part in the removal of 

 substances which, if allowed to accumulate and putrefy, 

 would become noxious to life. 



The feeding habits of another common Polychaet may 

 be selected for description in view of the interesting contrast 

 they provide to those of the lug- worm. This worm, 

 Cirratulus tentaculatm (Flattely, 191 6), occurs buried in 

 sandy mud beneath stones in shore pools and elsewhere, 

 its presence being indicated by a group of delicate rosy 

 filaments which usually display a certain amount of move- 

 ment. Unlike the lug-worm, Cirratulus does not live by 

 passing sand through the gut for the sake of the contained 

 organic matter, but selects the nutritive food particles, 

 algal spores, diatoms and general organic debris, outside 

 the body. The selection is effected by a pair of muscular 

 flaps, covered with sensory epithelium, which project down- 

 wards from the dorsal surface of the peristomium and 

 curtain off the entrance to the gut proper from an external 

 vestibule. Feeding would seem to take place by a kind of 

 suction ; the sensitive edges of the flaps admit only the 

 smallest food particles and these are then wafted backwards 

 by the cilia of the gut epithelium. 



Tube-worms are dependent for their food upon what is 

 conveyed to them by currents set up by the cilia on the gill- 

 filaments {Sabella, Pectinariuy Sabellaria, Serpula, etc.). 

 Nereids, on the other hand, have a protrusible pharynx 

 studded with chitinous teeth and provided with a pair of 

 powerful terminal " jaws." They are extremely aggressive. 



