HABITS AND STRUCTURE OF ARENICOLA MARINA. O 



The amount and value of the work done by lugworras 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 measure- 

 ments 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 1911 tons, 

 representing, 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 pros- 

 tomium, and is followed by a short achaetous portion (fig. 1, 

 MET), which in many specimens appears to be composed of 

 four annuli, divided, however, by secondary circular markings. 

 The first chsetigerous aunulus is produced into a strongly 

 marked ridge, just behind which the notopodial setae {Chn}) 

 are inserted, the corresponding neuropodia [Nm}) being very 

 short and containing only a few setse. Tlie intervals between 

 the cha^tigerous annuli are subdivided into rings, of which 

 there are, in the " Laminarian^^ variety, 2 2 4 4 4..., and 

 in the littoral variety 2 3 4 4 4 . . . respectively. 



The chaetigerous annuli do not mark the true somites into 

 which the body is divided. From a consideration of the 

 internal anatomy (see p. 10) we have reasons for believing 

 that, in the middle region of the body, the second groove 

 behind each chsetigerous annulus marks the boundary between 

 the somites. A somite is, therefore, composed of a chsetigerous 

 annulus together with three annuli in front of, and one behind, 

 it. The parapodia are not situated at the beginning, but 

 slightly behind, the middle of the somites to which they 



