ANNELIDA. 



141 



have come into existence till the close of the Cretaceous 

 period ; but it is found in great abundance in the London Clay 

 (Eocene) and the Crag (Pliocene). 



ORDER ERRANTIA. The Errant Annelides are character- 

 ised by the fact that the body is furnished with lateral tubercles 

 carrying tufts of bristles. The animal leads a free life, and is 

 not confined to a tube. The gills are placed along the back or sides 

 of the body ; hence the name of " Dorsibranchiate Annelides" often 

 applied to this order (fig. 91). 



Fig. 91. Errant Annelides. A, Hairy-bait (Nephthys) ; B, Sea-mouse (Aphrodite] ; 

 C, Lob-worp (Arenicola). (After Gosse.) 



Possessing no hard structures, the Errant Annelides are 

 hardly capable of being preserved in a fossil state. Certain 

 fossils, however, have been supposed to be the bodies of these 

 animals in a petrified state ; but it is very difficult to believe 

 that such is the true nature of the remains in question, and 

 it is almost certain that they are really nothing more than 

 the " tracks " of sea-worms. On the other hand, the Errant 

 Annelides are abundantly represented by their trails upon old 

 sea-bottoms or their burrows in sand or mud ; remains of which 

 occur in all formations, almost, from the Lower Cambrian up 

 to the present day. 



The burrows of Annelides, as a matter of course, run in a 

 direction more or less opposed to the surfaces of the laminae 

 of the rock, being often quite vertical. Sometimes such bur- 

 rows are hollow, but they are more commonly filled up by the 



