310 



ANNULOSA. 



^_v>- 



Fig. 180. Tubicola. a, Serpula con- 

 tortuplicata, showing the branchiae and 

 operculum ; b, Spirorbis communis. 



on or near the head, generally in two lateral tufts ; hence the 

 name of " Cephalobranchiate Annelides," applied to this order 

 (fig. 180). 



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 par- 

 ticles cemented together by a 

 glutinous secretion from the body 

 (Terebella) ; or it may be simply 

 membranaceous or leathery (Sa- 

 bella). Sometimes the tube is 

 free and non - adherent (Pectin- 

 aria) ; more commonly it is at- 

 tached to some submarine object 

 by its apex or by one side (Ser- 

 pula and Spirorbis). Sometimes 

 the tube is single (Spiroriris) ; 

 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 resem- 

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

 and Dentalium. In the living state it is easy to make a dis- 

 tinction between these, for the Tubicolar Annelides are in 

 no way organically attached to their tubes, whereas the Mol- 

 luscs are always attached to their shell by proper muscles. 

 In the fossil condition, however, it may be very difficult to 

 refer a given calcareous tube to its proper place. As a 

 general rule, however, the calcareous tubes of Annelides, 

 such as Serpula, are less regular and symmetrical than those 

 of Vermetus, whilst the latter is partitioned by shelly septa, 

 which do not exist in the former. Again, the tube of Den- 

 talium is open at both ends, whereas it is closed at one 

 extremity in the Serpulce. In the Annelidous genus Ditrupa, 

 however, the tube is open at both ends, so that this distinc- 

 tion is one not universally applicable. 



Tubicolar Annelides are known from the Silurian rocks 

 upwards, almost every great period having representatives of 



