ANNULOSA I ANNELIDA. 



21 



Tubicola are unisexual, and the young pass through a meta- 

 morphosis. 



The protecting tube of the Tubicolous Annelides may be 

 composed of carbonate of lime (Serpuld), of grains of sand 

 (Sabellaria), or of sand, pieces of shell, and other adventitious 

 particles cemented together by a glutinous secretion from the 

 body (Terebelld) or it may be simply membranaceous or lea- 

 thery (Sabella). Sometimes the tube is free and non-adherent 

 (Pectinarid) ; 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 pseudo-haemal 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 fila- 

 ment 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 con- 

 tractile power of the gills them- 

 selves. From the position of 

 the branchiae upon, or near, the 

 head, the Tubicola are often 

 known as the " cephalobranchiate " Annelides (fig. 79). 



Reproduction in the Tubicola is generally sexual, the sexes 

 being in different individuals ; but spontaneous fission has also 

 been observed. As regards their development, the process has 

 been thus described, as it occurs in Terebella : The embryo, 

 which is at first a free-swimming, ciliated body, "lengthens, 

 and the cilia, which were at first generally diffused, become 



Fig. 79. Tubicola. a Serpula con 

 tortuplicata, showing the branchiae 

 and operculum ; b Spirorbis com- 

 munis. 



