90 F. W. GAMBLE, M.SC, AND J. H. ASHVVORTH, B.SO. 



this. The post-larval stage, however, appears to he, for a short time, 

 pelagic (Benham, 1893). 



The curved burrow of the shore lug-worm is formed by the combined 

 action of the pi'oboscis, the swollen anterior region of the body, and 

 the waves of muscular contraction which pass along the body from 

 behind forwards. When the proboscis is everted and pressed into the 

 sand, the prostomium is slightly retracted into the body. The proboscis 

 is withdrawn full of sand, again everted, and the body is thrust forward, 

 partly by contraction of the longitudinal muscles, partly by a peristaltic 

 wave produced by the circular ones. The anterior end is in this way 

 rendered swollen and tense, and is able to enlarge the burrow, and thus 

 a passage is gradually eaten through the sand, smoothened by contact 

 with the skin, and lined by the mucus secretion of the epidermis. The 

 gill-region being narrower than that which precedes it, is thus, to a 

 certain extent, protected from friction, while, as if to ensure this, the 

 notopodial pencils of bristles are directed so as to protect the gills. 

 After burrowing vertically downwards for a depth of from one to two 

 feet, the worm forms a horizontal or oblique gallery, and then a second 

 vertical one which ends at the "countersunk" hole, through which 

 the anterior part of the worm may protrude, and so bathe the gills in 

 fresh sea water. 



The amount and value of the work done by lugworms has been 

 estimated on the shore of Holy Island by Mr. Davison (1891), and has 

 also been adverted to by Mr. Hornell under the name of " cleansing of 

 the littoral." Mr. Davison finds that the castings are larger and 

 more numerous above than below half-tide; and as the result of several 

 estimates and measurements he calculates that on the Holy Island 

 Sands, the entire layer of sand, to a depth of two feet, passes through 

 the bodies of the lugworms which live in it, once in twenty-two months, 

 and that in a year the average volume of sand per acre, which is 

 brought to the surface in the form of castings, is 1,911 tons, represent- 

 ing, when spread out, a layer of thirteen inches in thickness over the 

 surface of the sands. 



2. External Features. 



Segmentation. — The body is divided into an anterior chsetigerous 

 portion, a middle branchial one, and a posterior caudal region or tail. 

 The first region begins with the prostomium, and is followed by a short 



