ANNULOSA : ANNELIDA. 



265 



particles cemented together by a glutinous secretion from the 

 body (Terebella); or it may be simply membranaceous or leath- 

 ery (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); 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 

 organically attached to their tubes, whereas the Molluscs are 

 always attached to their shell by proper muscles. 



The pseudohasmal system has its usual arrangement, and 

 the contained fluid is usually red in colour, but is olive-green 



in Sabella. The respiratory or- 

 gans are in the form of filament- 

 ous branchiae, attached to, or 

 near, the head, generally in two 

 lateral tufts, arranged in a funnel- 

 shaped or spiral form. Each 

 filament is fringed with vibrating 

 cilia, and the tufts are richly sup- 

 plied with fluid from the pseudo- 

 haemal system. There is no spe- 

 cial apparatus required to drive 

 the blood back to the heart, but 

 this is effected by the contractile 

 power of the gills themselves. 

 From the position of the branchiae 

 upon, or near, the head, the Tu- 

 bicola are often known as the 

 " cephalobranchiate " Annelides (fig. 130). 



Reproduction in the Tubicola is generally sexual, the sexes 

 being almost invariably in different individuals ; but fission has 

 also been noticed to occur. As regards their development, 

 the young pass through a distinct metamorphosis. The larvae 

 (fig. 131, A and D) are freely locomotive, furnished with eye- 

 spots, and swimming actively by means of cilia, which are 

 principally aggregated into two rings or circlets, one placed 

 on the head, the other at the hinder end of the body. The 

 tentacles are developed at an early period, and the larva un- 

 dergoes segmentation. Finally, the cilia disappear, the larva 

 becomes stationary, and the protective tube of the adult is 



x^>> 



Fig. 130. Tubicola. a Serpula con- 

 tortuplicata, showing the branchiae 

 and operculum ; b Spirorbis corn- 

 in unis. 



