482 ANNELIDA. 



are truly the burrows of Annelides. The somewhat problematical 

 fossil upon which the genus Histioder??ia is founded is described 

 as a curved burrow, from one to nearly four inches 

 in length, terminating in a trumpet-shaped open- 

 ing, which is placed in the centre of a small 

 mound. The genus Arenicolites, again (fig. 347), 

 includes small double burrows, which form loops, 

 shaped like the letter U, opening on the surface 

 Fig. 347. -Burrows by two apertures placed close to one another. 



of Arenicolites didy- ,-p,, - - - -, - . . . 



mus. From the Long- 1 ne mouths ot these burrows are thus placed in 

 brmn). (L ° wer Cam " P^s, one orifice being supposed to be an aper- 

 ture of entrance for the worm, and the other one 

 of exit. Burrows of this nature occur abundantly in the Lower 

 Cambrian strata of the Longmynd, and are also far from uncommon 

 in deposits of Ordovician age. 



II. Wandering Burrows. — Various recent Annelides, among 

 which the common Lug-worm (Arenicola fiiscatoruni) is a notable 

 example, form long, wandering, irregular, and tortuous tunnels in 

 the sand of the sea-shore, at a little distance below the surface. In 

 these cases the worm subsists upon particles of organic matter dis- 

 seminated through the sand or mud, through which, therefore, it 

 literally eats its way. The burrows thus formed are, consequently, 

 very irregular ; they principally have a horizontal direction ; if 

 formed by many individuals, they may cross or intersect one 

 another in various ways ; and as the worm proceeds on its course, 

 they become filled up in the rear of the advancing animal by the 

 sand which has been passed through the alimentary canal, some of 

 this sand being often voided at some point at the surface to form 

 the tortuous "worm-casts," with which every wanderer on the sea- 

 shore is so familiar. Bodies which we may reasonably assume to 

 be of essentially the same nature as the filled-up " wandering bur- 

 rows " of worms like the living Lug-worm, are well known to all 

 workers amongst the more ancient, muddy and sandy strata of 

 the earth's crust, and they have received various names, and have 

 had very various origins ascribed to them. They usually present 

 themselves as irregularly cylindrical, worm-like elevations of the 

 surfaces of the strata (fig. 348), which usually are more or less 

 parallel with the laminae of deposition, but often run somewhat 

 obliquely to these, so as to thread successive laminae to one 

 another. Generally, they differ slightly in texture and colour from 

 the surrounding rock — as can well be supposed, if their origin be 

 as above described — and from their somewhat superior hardness, 

 they resist disintegration by weathering, and thus come to stand 

 out prominently above the surface. Though they may be much 

 matted together, and may thus appear to branch, it is only in some 



