ANNELIDA. 317 



fossil remains (such as Nereites and Phyllodocites) as being of 

 this nature ; but no structure has ever been detected in the 

 bodies in question ; and till some adequate explanation can 

 be given of the process by which it is possible for the soft 

 body of an Annelide, lying on the shore or in shallow water, 

 to be replaced by an accurate mould in mud or sand, it is 

 best to believe that these fossils are really of the nature of 

 " tracks." Very numerous remains which have been referred 

 to Errant Annelides have been noted as occurring throughout 

 the entire geological series, wherever we meet with muddy 

 or sandy deposits ; but the true nature of many of these is 

 still in the highest degree uncertain. Some are, perhaps, 

 really referable to the vegetable kingdom ; others are almost 

 certainly formed by Molluscs, or by Crustaceans ; others are 

 of entirely dubious affinities ; while others are, doubtless, 

 really due to the operation of Errant Annelides. It may be 

 added that the fossil remains which have been referred to 

 Nemertean Worms cannot at present be separated, in any 

 satisfactory manner, from those formed by Errant Annelides. 

 Thus the so-called Nemertites of the Silurian is just as likely 

 to be Annelidan as Nemertean, and the nature of the Legno- 

 desmus of the Solenhofen Slates is wholly problematical. In 

 fact, the entire subject of the remains of fossil Errant Anne- 

 lides is one of the most obscure and difficult with which the 

 palaeontologist is called upon to deal ; and all that can be 

 done here is to glance at some of the leading points of in- 

 terest connected with it, under the following heads : 



I. Burrows of Habitation. Various living Annelides live 

 buried in the sand or mud, between tide-marks or in shallow 

 water, and communicate with the surface by means of a 

 perpendicular shaft or burrow. Such shafts may, for con- 

 venience' sake, be termed " burrows of habitation," though 

 the animal forms a fresh one at will, as it moves from one 

 spot to another ; and, as a matter of course, they run in a 

 direction more or less opposed to the surfaces of the laminae 

 of the rock, being often quite vertical. Sometimes such 

 burrows are hollow, but they are more commonly filled up 

 by the matrix of the rock. The most important genera 

 which have been founded upon remains of this kind are 



