POLYCH.ETA. 



471 



trails upon the soft sand or mud of the sea-bottom. In this way 

 we know that the Polychaetous Annelides commenced their exist- 

 ence at least as early as the Cambrian, while obscure traces of 

 their presence have even been detected in the Laurentian period. 

 Owing to their comparatively frequent occurrence as fossils, it is 

 necessary to study this group of Worms in greater detail. 



Polvch^eta. 



Sub-order I. Tubicola. — The Worms included in this sub- 

 order are distinguished by the fact that they inhabit variously 

 formed tubes, to which they are not organically connected, and in 

 which they can move freely by means of their setigerous foot-tuber- 

 cles. Owing to their possession of an investing tube, branchiae are 

 only developed in the anterior region of the body (fig. 333)1 this 

 being the only part which is ordin- 

 arily exposed to the action of the 

 sea-water ; hence the Tubicola are 

 sometimes called the "cephalo- 

 branchiate " Annelides. 



The protecting tube of the Tubic- 

 olous Annelides may be composed 

 of carbonate of lime (Serpula), of 

 grains of sand (Sabel/aria), or of 

 sand, pieces of shell, and other ad- 

 ventitious particles cemented to- 

 gether by a glutinous secretion from 

 the body (Terebella) ; or it may be 

 simply membranaceous or leathery 

 (Sabelld). Sometimes the tube is 

 free and non-adherent (Pectinaria) ■ 



more commonly it is attached to some submarine object by its 

 apex or by one side (Serpula and Spirorbis). Sometimes the tube 

 is single {Spirorbis, generally) ; sometimes the animal is social, and 

 the tubes are clustered together in larger or smaller masses 

 (Sabellarid). 



When the tube is calcareous, it presents certain resemblances to 

 the shells of some of the Molluscs, such as Vermetus and Den- 

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

 these, for the Tubicolar Annelides are in no way organically 

 attached to their tubes, whereas the Molluscs 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 Annel- 

 ides, such as Serpula, are less regular and symmetrical than those 



Fig. 333. — Tubicola. a, Serpula con- 

 tortuplicata, showing the branchiae and 

 operculum ; b, Spirorbis communis. 



