POLYCH^TES 



257 



Type of PoLYCH^zETA. The Lob-worm (Arenicola 



marina) 



Habits. — On the flat sandy beach uncovered at low tide, 

 the " castings " of the lob-worm or lug-worm are very 

 numerous. There the fishermen seek the worms for bait, 

 and have to dig quickly, for the burrowers retreat one to 

 two feet into the sand. The burrows are curved tubes, 

 lined by a yellowish green secretion from the animal's 

 epidermis, and the surrounding sand is often discoloured by 

 some change which the secretion eff'ects on the iron oxides 

 and other constituents. The tubes are at first vertical. 



Fig. 138. — Arenicola marina. 



Entire animal viewed slightly from left side. Note anterior jnouth ; 

 setae on anterior region ; setae and gills on median region ; thinner 

 tail region often longer than shown. 



afterwards oblique or horizontal, and then turn vertically 

 upwards again. 



The lob-worm burrows like the earthworm, not only 

 forcing the anterior part of its body onwards, but eating the 

 sand for the sake of the organic particles and small organisms 

 which it contains. The sandy castings, which pass from 

 the end of the food canal, and are got rid of at the mouth 

 of the tube, fall into spiral coils. It has been calculated 

 that in a year the average volume of sand per acre thus 

 brought up in castings is about 1900 tons, representing a 

 layer of 13 in. spread out over the surface. This work, 

 comparable to that of earthworms, tends to cleanse the sand 

 and to reduce it to a finer powder. When getting rid of the 

 casting, the worm lies with its tail upwards and its head 

 downwards, or with its body bent like a bow ; when the 

 tide comes in, the mouth may protrude at the other end of 



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