ANNELIDA. 



137 



(viz., the Tubeworms or Tubicola, and the Sand-worms or 

 Errantia) which come under the notice of the palaeontologist, 

 and neither of these requires much notice. In both orders, as 

 throughout the division, the integument is more or less soft, 

 and there are no internal hard structures ; hence it is more 

 than doubtful if we have any example of the fossilised body of 

 these creatures, though such have been alleged to occur. The 

 Tubicolous Annelides, however, protect themselves by a tube 

 of lime, sand, or adventitious particles, and these investing 

 tubes are often preserved in the fossil condition. The Errant 

 Annelides, again, have left traces of their past existence in the 

 form of filled-up burrows or meandering trails upon the soft 

 sand and mud of the sea-bottom ; and from these we know 

 that the Annelides commenced their existence at least as early 

 as the Lower Cambrian period, obscure traces of their presence 

 having been even detected in the Laurentian Series. 



ORDER TUBICOLA. The Tubicolous Annelides are distin- 

 guished by the fact that the body is protected by a tube within 

 which the animal can withdraw itself by means of tufts of bristles 

 carried on the sides of the body. The gills are placed on or near 

 the head, generally in two lateral tufts ; hence the name of 

 " Cephalobranchiate Annelides" applied to this order (fig. 86). 



The protecting tube of the 

 Tubicolous Annelides may be 

 composed of carbonate of lime 

 (Serpula), of grains of sand 

 (Sabellaria), or of sand, pieces 

 of shell, and other adventitious 

 particles cemented together by 

 a glutinous secretion from the 

 body (Terebella) ; or it may be 

 simply membranaceous or lea- 

 thery (Sabella). Sometimes the 

 tube is free and non-adherent 

 (Pectinaria) more commonly it 

 is attached to some sub-marine 

 object by its apex or by one 

 side (Serpula and Spirorbis], 

 Sometimes the tube is single 



(Spirorbis) ; sometimes the animal is social, and the tubes are 

 clustered together in larger or smaller masses (Sabellaria). 



When the tube is calcareous, it presents certain resemblances 

 to the shells of some of the Molluscs, such as Vermetus and 

 Dentalium. In the living state it is easy to make a distinction 

 between these, for the Tubicolar Annelides are in no way 



Fig. 86. Tubicola. a Serpula con- 

 tortuplicata, showing the branchiae and 

 operculum ; b Spirorbis communis. 



